offtopic question
How long you need to get the money back if you buy some s7?!
If you mean the return on investment, I would imagine that would be dependant on the price of bitcoin. Dipping to low you would not see the light of day making your money back on a purchase of one.
BTC purchase (4BTC) to 4 BTC ROI is 5 month. Thats not counting deduction based on electricity cost, downtime, shipping cost and time and cost of PSU, etc. And the S7 on bitmain is sold out, otherwise at the lower BTC price, 4BTC is not too bad.
However i still think its an expensive pick.