It depends. Some folks put on muscle easily. And others use chemical enhancements. And some fit both camps.
I've known a couple of dudes that put on 30 pounds in a year. Unless you have the heart of an elephant, you can't keep that up for too many years before you're gassing hardcore.
Most folks, though, can put on at least 10 pounds per year. They won't hit their limit overnight, but they certainly can hit their cardio peak - especially if they aren't doing pretty intense cardio work as well.
Defecato, ergo sum.
Leading the Thiago Silva bandwagon