I'm fairly lucky, in that I have my electricity included in my rent, because I have electric heat, so the heat from the PCs and GPUs offsets the heat needed from the electric heaters. (Basically, they don't come on.)
Once it gets warmer out, though, I think that I probably won't mine, on anything older than a 14nm card, due to power-consumption issues, and needing to run the A/C. Which will probably cut revenue in half.
Thus far, though, I've basically paid for my GTX1070ti, and 8GB RX 570 Gaming X that I picked up, and partially paid for my prior RX 570 4GB card and HP Power Gaming pre-built PC. (That included a GTX1060 3GB card, in Dec., for $500 + tax B&M.)
I figure I'll continue through April on most of my cards, and possibly into May, on the GTX1070ti and maybe the RX 570 cards. Then I'll probably have made ROI.
I may not have become rich, but hey, a few "free" (well, it took some work) GPUs isn't bad. Of course, by then, NVidia will have new cards out, so even if I wanted to sell them, they wouldn't be worth much, I expect.