in reply to How should a timed session be implemented?
As BUU as mentioned HTTP does not lend itself to this type of business procedure, and as so, a re-think of the goal of your project is on the cards.
The only way of doing this is, each page view issues a token, and takes the delta from the previous view. If the delta is > n then you start/stop/reset your timing calculation.
This is ultimately flawed as it relies on the user keeping their cookie, if a user throws it away, then the process will fall apart.
Probably the best way to make cash off the web is subscription.
Think about this: Of your extensive travels of the internet (web), which is a well mature product, how many sites have you come across that use a time based algorithm to make money, and how many subscriptions sites have you seen?