For my Birthday, I and five friends held what I called a "Kitchen Sink" Game. EVERY DECK I and my friends owned at the time plus ALL the extras we owned at the time. 17 different Munchkin decks (we split Zombies in two and Original Munchkin in three for an even 20 piles).
Here's a Twitter post I made about it.
https://twitter.com/MisterH37/status/848671799734939648
It took 5 hours and ended, not when someone reached Level 20, but when 7 seals were opened.