Best thing to do is consider a sweet spot of resale value. Recently when the price rose people were paying stupid money for ASICMINER USB's and i was able to sell some for more then i paid for originally regardless of BTC mined. Sold a BFL Jally for almost what it cost me and that had been mining for 9 months, otherwise the switch off point is when it costs more to run unless you are prepared to pay out of pocket for convenience of mined coins versus using an exchange.