"Expiring" transactions?

Is there a way to make transaction automatically COMMIT or ROLLBACK after
certain period of time?
I want website visitors to be able to page through search results using
single cursor. Since users can leave the site at any time, I can't know when
to end their transaction.

I guess I can write a broker that gives each user a connection and tracks
when he's gone, performing necessaty cleanup, but isn't there a simplier
way?

Best regards,
Egor Shipovalov.