Let's do a simple calculation.
333MHash/s can make BTC0.12 per 14-day now. Assume the difficulty increases 13% each time (every 14-day).
You make BTC0.12*(1-0.885^26)/(1-0.885)=BTC0.9999 in one year (a geometric sequence with common ratio .885 and 26 terms).  you can break even in one year if the difficulty increases slower.

Last two times, the difficult increases 28% and 24%.
...