Looking for a mathematical formula for compound investing over a certain amount of time.

Title is basically it, did some research all I found was compound interest formulas which, while they worked, gave massively inflated answers that I was unsure of.

I know there is no set monthly/yearly return on stocks so of course all figures I get about are taken with a pinch of salt, but I am bored at work and am trying to work out when I can (roughly) retire through this investing.

Thanks.