For a 0.335 GH/s device to recover 0.17BTC with only a 20% diff each change ... it would take ... 190 Days
... at 1000 days it is ... 0.17936377 BTC (so pretty much never reach 0.18 BTC)

If on the other hand it was 30% diff each change ... at 500 days it will have generated ... 0.11957525 BTC ... 1000 days ... 0.11957587 BTC ... so it would never reach 0.12BTC ... of course 0.5W isn't free either ... especially after 1000 days ...
http://www.tradebtc.net/bitcalc.php

For a 10GH/s device at 20% increase each change, it would take 89 days to get 4BTC
at 30% increase it would ... 500 days 3.56940335 BTC ... 1000 days 3.56942180 BTC ... so never reach 3.6 BTC
But the electricity is pretty relevant also ...

Yes many people buying AsicMiner hardware will not get their many back ...