I keep seeing people talk about how long "ROI" takes. What does that mean?

If you invest in something and get a profit of even 0.00001% per day, isn't that still a return? Aren't you making a return on your investment from day 1?

I understand if you're buying something, like a mining contact say, which takes 100 days to pay you the amount you paid for it. Then you're running at a loss for the first 100 days, and at a profit after that. But isn't that a special case? If I buy Apple shares, I'm investing in Apple. Does it make sense to talk about "how long for ROI"? If the shares go up the next day, I've made a return immediately, haven't I? It's the same with Just-Dice, and as far as I know all other dice sites with crowd sourced bankrolls. You can invest and divest at will, with no fee, so you can "ROI" in 5 seconds if you're lucky.