There are two approaches for the timer here. First, using a virtual momentary button with a defined auto-revert interval which can be used as a trigger to evaluate whether the light is still on and turn it off. Second, would be to use the Edge Counter Utility from @TAustin to count elapsed time and then at some selected value (say 15 mins) then turn off the light.
You can combine one of those options with motion conditions to set up your routine.