*Never* get your money back? That would be hard based on the devices efficiencies. Would require high Elec prices, dropping BTC value, skyrocketing difficulty, late delivery, and low Hash/J+$/Hash ratios.
A long ass time to get your money back? That's very probable.
Tons of calculators out there, bitcoinx.com,
http://bitcoineering.appspot.com/simulator,
http://tpbitcalc.appspot.com/, etc.
Plug in numbers that sound right to you and then have a think about it. No one really knows what will happen, so your guess is good as anyone elses.
That dude above me sounds reasonable, but he could be totally wrong.
