Then I found a simple solution. The timeout attribute should be given in web.config of the site in question. Open the web.config of the site, and look for the tag
Below this, there is ought to be a
Timeout is given in minutes.
For other settings to be done when having forms authentication for a MOSS/WSS site, go through this post:
Additionally, for cookies to be retained in forms authentication enabled subsites, install SP1 on IE 6.0, otherwise they would not be retained and on opening any subsite, user would be prompted again for logging in.