The weight of your bag will be determined by the use you intend to put it to.  If you are after a bag that you can stand a whack the hell out of, you need a bag that weighs around 30 kg (66lbs). On the other hand, if you want a bag that is going to move with you, requiring you to constantly adjust your footing, move around and be more agile, go for a lighter bag that is filled with rags and weighs up to 20 kg (44 lbs).