Waiting until a function finishes?

This is probably a major noob question, but when I call a function (such as to fade the screen to black) from my main, the rest of the code continues instantly. How do I make it wait until the screen finishes fading before continuing? I could stick a wait after the function but I'd like something a little more reliable if it's going to take different amounts of time.

wait_for(function)

So simple! Thanks!