How to Determine if a Group Has been on longer 10 Secondthan s

[sjfp]

Trying to find some code that can be used to determine if a group object has been on from more than 10 seconds. but can be expanded to more than 5 mins if needed.
Will be used to switch fans on in en-suite if lights have been on for a while.
Could someone point me in the right direction, or if there is a sample  code already done.

any help will be very appreciated