I found this info on a website:
Truffle Oil
Because of its strength, only a few drops of truffle oil are required to add the earthy truffle flavor to a variety of foods, such as meat, fish, pasta, risotto, salads, and sauces. Truffle oil may lose some of its intense flavor and aroma if it is stored for long periods of time.
It was pretty vague and didn't give an exact timeframe as to when it expires, but oil does go rancid after a while and if it loses its flavor, a new bottle may be in order after all.
ETA: I also found this page here:
http://www.ochef.com/93.htm and it does say you should use it by 3 months.