Prep time is useful but not necessary. In fact, so is a GM. I've been in fun adventures where we had neither a GM nor a fixed plot. Not quite as great as a proper campaign but fun nonetheless and very quick to set up. Now, there are games where you need a GM who plans things out in advance - Shadowrun, for instance; at least if you play it the usual way with legwork and so on. But if you don't do anything that requires meticulous advance planning you can really just pick characters, have someone come up with an initial situation and make the rest up as you go, passing GM duty from player to player as people come up with an interesting direction to move the story to.
Now, I can't say I've done that with D&D; being German I prefer The Dark Eye*. But we did do it with a few systems including TDE, Shadowrun and a weird TDE/Shadowrun/Star Wars/Exalted/homebrew crossover game where we constantly had to translate back and forth between systems. Good times.
tl;dr: Roleplaying can take a lot of prep time but if you're willing to have a more chaotic experience you can do without it - and even without a GM.
* D&D doesn't even have crucial skills like Pottery, Housekeeping or Crystal Growing. It's a wonder anyone can get any adventuring done with that system.