Your best bet is to install Linux (Ubuntu or whatever) and use it as your daily use OS. When you find yourself comfortable working in it, doing all the same tasks you do normally under windows and a bit more then you're ready. As for how long it will take, that's totally up to you.
Personally I worked with *nix (Solaris and Red Hat) for a anywhere from 2 to 16 hours a week for a year before I would classify myself as even a 2 on a 5 pt scale.
0 - Sub-Noob (Linoox? Lynux? what's an ISO?)
1 - Noob (wtf is ls? ps -ef .... is that a bad word? wtf is a commandline?)
2 - Not a Noob (I can ls, ps, cut, grep, pipe output, and script a bit and I know why #/usr/bin/girl is funny and understand why there is no place like 127.0.0.1. I get what $PATH and $LD_LIBRARY_PATH are and why they're significant.)
3 - Intermediate (I can comfortably get around in a few different Linux GUIs, I can update the OS without breaking things too badly and recover if I do. My boxen send me alerts when things are busted or breaking. My friends sometimes call me with *nix questions. I admin a few boxes, email flows, web pages are served, etc.)
4 - Expert (I have my own distro...no one uses it but me but I did it just to say I had. I make my own patches/modules/installers. I contribute back to the OS community.)
5 - Professional (Got root? I have my own Beowulf cluster.)
5+ - /me is god, my cluster made the top 500 list
Currently I'd say I'm a 2.5.