That is not done through Security Assist, it is controlled with the Session Time out settings on the server.
see this post from the Webmaster world forums:
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.