yeah what BT said...For the vegetative growth phase the most commonly used light schedule is 18/6. Some people will uses a 20/4 or a 24 per hr. per day. 18/6 always works well for me.
When you put your plants into flower would be when you would switch them to 12/12.
The most common three-prong timer I've seen is an Intermatic Heavy Duty timer, which can handle 15A - enough for a HID. Any light duty timer I've seen is two pronged and a non-starter for an HID user.if you are using a HID light you need a timer made for them because a standard timer cannot handle the electrical load.if you are using a cfl light i dont know if a standard timer can be used because i have never grown using cfls.