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.
Haha I did same. I had 30 x 6 card machines at the time and hit zec as soon as it went live and sold it as soon as it hit my account. Man those were the days..., People were FOMO hard on Zec - Supply and Demand baby, it's a beautiful thing.
I also remember those days, but I happened to be unlucky and couldnt find a working pool that processed withdrawals in time as a lot of pools either delayed withdrawals or didnt let you withdraw immediately at all, so I didnt make nearly as much as you guys did. Still broke even after a while on a six card rig, which was the only GPU rig I had back then. A bit better than the gambling some people do today with mining an extremely low difficulty coin and hoping it goes up in the future, but such a situation likely wont ever happen again.