Come on over to the DIY subforum.
There have been quite a few threads on shelf-life/storage of nicotine, including this latest one
http://www.e-cigarette-forum.com/forum/diy-e-liquid/479021-shelf-life-nicotine.html
In short, stored in a dark, cool area, probably a year. In a freezer, longer. There isn't enough data yet to know just how much longer. At least one user reported nic taken out of his freezer after 3.5 years storage was perfectly fine.