Hellooooo out there!
I am truly an amateur on this. That said, i believe the amount of time you would like to set it for is based on the resister and capacitor supplying the Light-sensitive diode. You may use formulas and calculate, or have fun and just et a ball park, then tryout different combinations of Caps and Resistors.
I do not remember which, but the relationship between the Transistor and capacitor, one affects the voltage, the other affects the duration.