I am tring to drive a 12V DC motor for 10 to 15 seconds. My thought was to charge and then discharge a capacitor. I dont know what size capacitor would be required for that time and voltage? Or should I just use a resistor of xx ohms to limit the amount discharged?
That depends quite a bit on the motor, but it is doubtful that you would be able to store enough energy in any reasonable capacitor to run a motor for that amount of time. How about using a circuit that when triggered will just supply power to the motor for 10-15 seconds? That is much easier to do.