Cardarine gw is not a hormone or a stimulant so theoretically you could use it forever but I don't think using anything indefinitely is smart.
I recommend 16 week cycles of cardarine with 4 week breaks inbetween. Since cardarine does not effect testosterone levels in any way, you do not need to run a PCT after your cycle or anything like that.
The recommended dosage of cardarine is 20mg/day. The half life is 24 hours so it really doesn't matter when you take it. Just remember to take it each day. You can take it with food or on an empty stomach.
You can order cardarine from sarms1.com. They are the approved SARMS source of the board.