Providing things are operating normally, there's no limit.
On the main mail page, javascript runs in the background to periodically poll the server for new messages, this polling is enough to keep the session alive. I've had a session at work still active after the weekend.
If you're off the mail page for too long and not interacting with the server, the session will eventually timeout (not sure of the exact timiing).
Also, sometimes there's a problem behind the scenes which usually results in you being returned to the login page with a banner indicate the session is invalid or expired.
