FWIW, the question in the topic is confused. The question shouldn't be "600 GH/s how long is it good for?" it should be "0.6J/GH how long is it good for?" since absolute hashrate has nothing at all to do with hour long you can usefully mine with it, but power consumption sure does. Any answer on the lifetime will ultimately be mostly conjecture, but at least its a sensible question once framed in terms of power.
Talking about J/GH introduces another variable... BTC:USD price.. since power is billed in $. If BTC price suddenly for no apparent reason becomes $100000/BTC ... then i guess even mining on CPU would be cost effective in terms of J/GH