do you have any days off
do you have any day-offs
which one is correct
You just have to learn these things - sorry ;
day off -> days off
payoff -> payoffs
If you see them written down, there's no problem - add an S to the noun. But when you hear it, you have to hear the word-break (which is often not there).
It would be possible to derive a rule: a day off is a kind of day, but a payoff isn't a kind of pay - so if it's a kind of something, add the S to that word. But I think it's probably just as easy/laborious to simply learn the uses.