Depends on what you are willing to spend. Antminer S1 are 1.45BTC right now, in Slush's pool (which is having crappy luck), I get .006 per block and about .02 - .05 per day depending on luck. That will put me at about 1.5 months to hit even. I wouldn't go anything than a S1 right now. Price is too good to get anything else.