Setting a domain to stop logging people out?

[TogaDave]

There's an account on my server, a member based service where people log in and upload photos through a web interface. When a user logs in to the site, it seems after about 20 minutes of no activity, it logs them out (kind of like a banking site would). As far as I can tell there is nothing in the site itself that would cause this. Does anyone know if this is a setting within the server I would need to change so that they can remain logged in for 60 minutes?

Thank you,
Dave

[Lyttek]

Could be a cookie expiration, perhaps?