I'll stop and sell my rig when 1) the net profit earned is not enough to pay for the entire rig within 1 year and 2) the value of my mined coin has slim of chance of ever rising within 1 year. When these 2 conditions are met, it makes more sense to just hardware now because depreciating costs, risks of hardware failure, and risk of market crash clearly outweigh the opportunity. At this point, selling the rig entirely would generate more revenue.