How long

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!

It can probably take a good number of months to make a map. My first "real" map that I'm currently working on has been in development since April (four~five months!) and it's not even close to being finished, but I'm a bit slow though.

Bear in mind that the first few maps you make will be more for practice. Hammer has a lot of tools, but they're easy to learn once you get the hang of it. Just takes time. Once you're comfortable w/ the editor and understand some of the finer points, making a quality map can take anywhere from a couple weeks to months. Really depends on how much free time you have, lol.