Year and 3/4, give or take. I mostly train by myself and with a few friends from my area who started with me, but I'm definitely the most serious about it.
I happen to be lucky and live near DC, which is a HUGE hub of activity. There's the first Parkour gym, most of The Tribe lives around this area, and there's a thriving community, dwarfed possibly only by Denver and NYC. So when I can, I meet up with people more experienced than I am, but when I can't, which is usually, I just train by myself, or with a friend or two.
Are you interested in starting? If so, let me know. I have a wealth of resources and can teach you a lot, even if I can't be there in person.