I had a temp ileostomy, but only for about 2-1/2 months (which was long enough, thank you), so I can't say about the hernia part. My surgeon reserved the OR for an hour, so I would think with your hernia repair and adding in some time just to be conservative, four hours would be a decent estimate.
I had to go through two colon preps, as I had a colonoscopy performed about two weeks ahead of the reversal. For the colonoscopy, they asked me to take the pills and then drink the magic potion. However, for the liquid diet, I had soup and lime Jello and by the time I was supposed to drink the magic potion, all my bag had was green liquid from the Jello. I called the GI doc and his nurse told me that I wouldn't need to do the rest of the prep.
For the reversal, the surgeon said that if Jello worked fine the first time, do the same thing. I did, except this time, the liquid was yellow because I switched over to lemon Jello.
For both, I did chicken soup. The first time, we got it from the store, but the second time was homemade. Thank you wifey!
Good luck with the reversal. I know the apprehension is going to be there, but once you're reconnected, I think you're going to be very happy. Just make sure you do a lot of walking and be prepared for some pretty good stomach gas the first couple of weeks.