I believe it's 2 months after release. I did a little searching and found that it's suppose to expire Feb 1st, but as long as you don't restart your system or the client itself it will continue to run. The expiration dosn't cause the client to stop, it only prevents it from starting after the expiration date. At that time you'll need to download the next Beta or the final release (which ever the case maybe) The FAQ said that another client would always be released prior to the expiration.
http://foldingforum.org/viewtopic.php?f=8&t=163