I gained over 50lbs (yikes!) but the majority of it was water weight. I lost the majority of it in the first 2 weeks, and a month after my daughter was born, I think I only had 10lbs left to go.
Everyone's different. Some people only gain a little bit, some gain a lot. Some find it easy to lose it afterwards (especially if you breastfeed), and some don't.
No matter what, though, it's worth it!
Good luck!