session.cache_expire will just decide what http expires header is sent for the PAGE, not neccesarily for the session itself.
the session cookie lifetime is controlled by
session.cookie_lifetime
deafult is 0, which the browser interprets as "hold onto this cookie until they close the browser"
if your having timout problems, you prob want to adjust the session.gc_maxlifetime value as well.
take a look at the manual for ini_set() and look at the various session directives, and the descriptions of what they do.
if you dont want to wait the whole 600 minutes for browsers to forget, i think you could prob set session.name to a new name and that would do it.