I have been on methotrexate for a little over 5 years. After official diagnosis and he fact that steroids were doing nothing for me and I was alergic to Pentasa they decided to try this. I take the oral tables 25mg a week which is considered high dosage which the docs aren't too happy about as it is quite a toxic drug but as its keeping me well at that level they're OK with me staying on it until I've finished uni etc and got my life together before trying to get me onto something else. I think its the female factor as well as having kids on this drug is a massive no no.
I started out on it on its own with folic acid once a week to reduce side effects and limit any damage, I have bloodtests once a month to make sure my livers not being damaged etc by it. It worked well on its own at first, within about 2 months my symptoms were greatly lessened and my inflammation levels had dropped but still wasn't total remission. It was a few months later when I was facing surgery they tried me on an infliximab infusion as well and thats what kicked me into remission and the methotrexate also works as a maintenance drug with infliximab and that has kept me very healthy the last few years.
The only side effects I have experienced were when I first started after the first couple of doses I'd have mini hallucinations for a couple of hours and see colours and shadows behind me - quite bizarre, but that stopped after a few weeks of taking it. To begin with my hair thinned a bit but that also stopped and other than that I have bad nausea with it so I take it at night to sleep through most of it and then the next morning I take prochlorperazine (quite a strong anti-sickness med, it took trying quite a few different ones until I found one that worked) to stop that.
So yes you can be on it for long periods of time but as long as you're closely monitored and its suitable for your situattion