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.