I started my finasteride 12 years ago, in 2004. I had a buzz cut at the time, so within a month I could tell my my balding crown was starting to fill in. At the 6 month mark, I think I reached my plateau and have stayed there ever since. My crown is a bit thin (especially when wet), but the rest of my head is full and thick.
My friend tried finasteride in 2005 after I was raving how well it worked for me. However, he was balding on the front and top of his head, not the crown like me. So unfortunately, after 6 months he didn't see any results and quit.