Not speaking from experience (just starting up myself), but from tutorials and articles I've read up on, a decent, properly functioning TF2 map can take a good few months to complete. Starting from conception and initial sketches, through layout, testing, fixes, detailing and more testing, up to the final release.
For an experienced mapper, I imagine it can take as little as maybe 6-8 weeks for a full map (possibly even less for smaller ones, such as Arena maps), while a newcomer might need twice or even three times that long.
Generally though, I believe a map is only truly finished when the creator is happy with it.
Good luck with your project, and have fun!
