I have typically done a single model release with my models, but they are folks I have a long-standing relationship with for the most part, so there is an understanding between us and a comfort level that mitigates the need for re-releasing every time. It is referred to as a "standing release", but it is probably better form to get a separate release each time, as Bill says above.
I concur with Bill. My releases are specific to the shoot and the model. Plus if the shoot requires ANY hint of sexual content, I get a copy of their government issued picture identification. Everytime.