How long does the contracts last for this then.It says "The contract expires at the moment that the principal has been paid back in full.
BITMAIN may pay extra Bitcoin to the owner of PACMiC before the principal has been paid back in full, which will accelerate the payout even more." but no one can tell me what this means,I don't get what it means by when the principal has been paid back in full
It means what you get back your initial investment. When you get back your 220 USD back, that s principal.
To avoid confusion, each day, they pay a little bit of your principal + your interest. When the sum all those small chunks of your principal which you get paid each day reaches your initial investment, the contract is terminated. You do get that interest as well.
If we are talking about PACMiC, interest is about 22% a year. So you ll get 220 USD back and 22% interest at the top of that. This is in ideal conditions.
If we are talking about S5 miner, ROI is about 300 days and after that, it s all profit. Again, in ideal conditions.