How long are blood thinners usually prescribed for?

Depends. It depends upon the reason for which they are prescribed. For example, after an uncomplicated venous thrombosis in the leg, the length of anticoagulant use should be about six months. For a mechanical heart valve, the blood thinners are continued for the life of the valve.
Depends. Depends on the cause. Sometime indefinatly for atrial fibrillation or 6 month for deep vein thrombosis. Only treating physician can determine the time.