you should roi in 3 months, assuming diff stay the same, and assuming your build will cost around $2k(6x390)
sounds about right, plus you will have resale value of 6 390s, which shouldnt be too bad even if the new cards come out. and after that its the end of ETH probably (for mining), but there will be more coins to mine
The PoW could finish later than expected. At first, we heard it is early next year, but it is delayed.
so after POW -- the POS (no mining) --- then it will be mining again after POS? like BTC?