Cookie expiration

Can anyone please offer some help and advice with regards to changing the period of validity for a session cookie?
Is is possible to change it such that on logout or closing a browser the cookie expires / dies?
If not can the period of validty fora cookie be changed such that it refreshes (say every 15 seconds).
Am sure it is written / explained somewhere but cannot seem to find it.
Thanks to anyone for their assistance.
Paul