Depends entirely on the software.
There could be two ways that a 'session time' is kept. Eithe set in a cookie or set on a disk file. Either one will have a expire default set to it.
You could attack it by:
- resetting the cookie with a false time/date
- extending the expirey time in order to stay logged in longer.
- manipulating the disk file time/date.
either way, it is going to be software dependent.