Well I actually ROI'd a GPU in less than 6 hours when the ZEC launches back in Nov 2016.

Back then the price was crazy high on Poloniex, basically one ZEC was worth more than one BTC and if you were on an actual pool that paid you could of been one of the first few deposits and made crazy gains for almost no work.

The issue with this is that we don't see these gains everyday. It was basically pure luck and it won't help you out in the future.