it seems the bonus is 50% no matter how long you gain rested exp. the amount you receive before that wears off is dependent on how long you rest. for instance i think it is about 500 bonus exp per hour of rest. so if u rest for 1 hour your rested exp bar will be 1500 exp long. so if u normally would have gotten 1000 exp for doing whatever you're doing you will get 1500