To be honest in my opinion I would not rest for much longer than 3 minutes as then I think you will be resting more than is needed and will see more slender results, as you will be training at a lower intensity, when it is not required. However, intrestingly I can find nothing on the differeing results observed through different rest periods between sets in resistance training. Therefore I cannot back up my opinion with any evidence so may very well be wrong. Hope that helps