I would not take Entocort for years, that seems reckless. My GI did say he feels it's okay to be on for months at a time though. I've been on it twice, for about 7 months the first time (Oct 2010 - April 2011) and for about 8 months the second time (Feb - Oct 2014). It's a bit slower acting than pred so it takes a bit longer for me to get into remission from it. And yes, as the others have said, it'll still leach calcium from you so it's probably a good idea to supplement, and if you're well enough to exercise, try to do some weight-bearing exercise as well to keep your bones strong (Entocort can also rob you of muscle mass, so that's another good reason to do some weight-bearing exercise if you're able to).
For me, I feel better on Entocort pretty quickly, within a week or two of being on it. But feeling better and actually being healed inside are two different things. That's apparently why it took me so long both times on Entocort, it just takes awhile for it to heal me inside to the point where I can taper down and then come off of it.
I did get one fairly bad side effect from Entocort, which was recurrent headaches (I would get bad headaches that would sometimes last for 5 days at a time, and at times I'd have 3 of those headaches per month - not fun). Apparently only a small percentage of people on Entocort will get headaches, so don't be too worried about that. And honestly that was the worst side effect I experienced, and I could deal with it although it wasn't wonderful. So hopefully your side effects will be mild and few.