Not sure where you're from. It seems like in other countries people stay in the psych hospitals longer. From my experience & others I've talked with around here, usually stays last from 3-10 days. Voluntary admissions are more likely to be on the shorter end. Involuntary commitments are likely to be at least 6 days. If they feel you have good support outside of the hospital & you are willing to participate in outpatient programs (usually Intensive OutPatient Therapy -- which I actually really liked & found helpful, it was 8 hours/day Mon-Fri).
If you have physical issues that have resulted from your self-injuring, it could be slightly longer.
Hospitals are not always a great place to stay. I can definitely relate to that, but there are some advantages. They can run a bunch of tests on you to see whether there is something physical that is causing or worsening your mental health issues. Thyroid issues, anemia, pernicious anemia (B12 deficiency), inflammation, infections, etc. can all cause issues & are routinely tested with all psych patients. In my case, we discovered I had hypothyroidism (which contributed to my weight loss, low energy, sleep disturbances & a couple dozen other issues), anemia (which contributed to low energy), pernicious anemia (which contributed to low energy, low mood & increased nerve pain) & low sodium (caused low bp & moderate dehydration -- again, low energy, plus nausea & sickly feeling).
I got some meds in the hospital. They talked to me about iron & b12 supplements, plus about drinking more water & eating some salty foods with it to make sure it gets absorbed. They also upped my anti-depressants (which we later found was a bad plan since anti-depressants lower thyroid function & so I got worse until I started on Synthroid) and suggested I talk to my GYN about starting BCP to address heavy bleeding contributing to the anemia.
The food honestly was pretty good (& normally I HATE hospital food). Most of the days are spent in group therapy, so there isn't much time except for communal meals to have conversations with other patients (plus, like Karen wrote, they pretty much all feel the same way you do).
Insurance does not approve private rooms, but usually if there are rooms available, staff will try to separate patients for as long as possible. As it gets closer to the holidays, though, more people become severely depressed, decompensate with other mental illnesses or make bad choices with drugs/alcohol, so it is possible you might share a room with one other patient of the same sex for part of your stay (they typically will do everything possible to not put you with another patient on the first night, though). The rooms are divided by a curtain like a regular hospital room, so you can get some privacy. As for snoring, I wouldn't worry about it. The staff comes by all night every 10-20 minutes with flashlights to check on the safety of each patient, so you probably won't really get sleep anyways, even if you're in a private room & everyone on the floor stays in bed the whole night & is super quiet.
If you sign yourself it, the stay is generally much shorter. It could be as short as 3 weekdays if you sign yourself in. If it's involuntary it will likely be at least 6 days. Usually, in the US at least, stays don't seem to be more than 10 days unless there are addiction to drugs/alcohol issues involved, or medical issues that need to be attended to prior to discharge. Most places won't keep patients for more than 21 days. Even those that do, it is very, very rare that someone would stay in-patient for that long. More commonly, they want to run a bunch of tests, observe you in group sessions, observe you eating & see that you are at least pretending to sleep (& almost everyone pretends because the flashlights just make it almost impossible for all but the very deepest sleepers). Also, they need someone to release you to. If you sign yourself in, you can usually pick one family member to be that person & you can call them to let them know. They will want to observe you talking to your family member to make sure there aren't any dangerous issues that are likely to creep up right away if they release you to him/her. If you are involuntarily committed, unfortunately, it seems they will more often call multiple people, schedule family sessions, etc.
Maybe a family session would help in your case. I don't know. But you should try to figure out someone who is family & an adult who you can trust to pick you up at the end of your stay. I've talked with a lot of people & I've never heard of someone being released to a boyfriend/girlfriend and they definitely won't send you home on your own.
They do schedule several group sessions per day. Usually the patients do very little talking. It is really set up more like a lecture. The only one I've seen that is different is art therapy. Movement therapy there isn't really much talking from anyone, including the teacher. So it's not really like regular group therapy sessions.
But honestly, I want to encourage you to try some new things. It sounds like you are so miserable right now. Unfortunately, depression without any intervention only gets worse over time. Same thing with cutting. I know you're afraid, but what do you have to lose? Life is already so bad. If you do nothing, things will just get worse. You are then likely to end up being involuntarily committed. You can try to fight against change in the hospital after being committed, but usually that means a longer stay. They will just keep repeating the same information over & over again every day until you start to accept it. The longer you wait to get serious help, the harder it will be. To sign yourself in voluntarily right now will probably be the easiest route to start on the road to feeling healthier. By doing that & putting up with telling one family member about being in the hospital, you will be in, get help & be released pretty quickly. Then you can continue your treatment as an outpatient.
I will say that things can get better. We just have to fight -- hard -- every single day. 3 years ago all I could think about 24/7 was suicide. I was miserable, paranoid & would do things to hurt myself. As I got more & more destructive, I lost much of what made up a life. I was so unhappy. Those around me were so unhappy. My job was suffering; my home was a hovel; it wasn't good. I was willing to try to at least just not feel anything & set that as my goal, but honestly, that was worse. My family was happier, but it was much more dangerous for me. After years & years of misery, I finally got help & decided to fight. I would tell myself every day "I want to live" "I don't want to ever try to take my life ever again". And little by little, it got easier. It was not a straight line forward, but I have been making good progress. I made a decision to make a set of changes & now I mostly just stick with them -- it is not that you will have to constantly keep changing everything you do all the time. And I've found that now I have more happy days than depressed ones or neutral ones. And when I am miserable, I can follow a plan & start to turn things around before they get too bad. I really like my life right now -- I can trust friends, I can focus at work, I volunteer, I keep my home reasonably picked up & I am strong enough to stay away from my highly critical family. And, wonder of wonders, I think I might be a pretty decent person who maybe doesn't deserve all the abuse I would shell out on myself (I kinda have a sneaky suspicion you might be a pretty alright person too & you deserve better than what you're having to live with right now:).
I share that in hopes that you too will find at some point that life is not all miserable. It is work to make the changes at first, but it gets easier each day & then before you know it your life is wonderfully different.
best wishes with everything,