Native session problem

[eluser]dimis[/eluser]
I do a cms for a e-commerce site with native sessions of php (session_start()).
How can I configure the site to terminate a session after some time?
Yesterday I close my PC and today it was as I were already log on

[eluser]dimis[/eluser]
I use now db sessions od CI and as I see I solved it.
Dimis