You'd be looking at about a 200-250 day ROI for any rig you build, price of electricity depending.
I wouldn't advise it.
here is for bitcoin, and you are looking with cheap electricity for only 5 months roi with an antmienr s9, bought in second hand and not new, but then you have the diff that increase, but you have also bitcoin value that increase
not much of a gamble anymore to take part in the miningactivity, i think it's doable, especially if the value will go very high after segwit
If the value is rising mining is profitable, that's correct, but buying and holding while it rises is just as profitable. Not much of a challenge there.
When the value is relatively steady, mining becomes much less profitable thanks to rising difficulty, but it's doable, just like you've said.
The problem starts when the value starts dropping. A holder can sell his coins almost instantly, but selling a miner is much harder.