With a wildly unround chainring, bent stays and damn near no dropout to tension with (semi horizontal), I ran an unideal chain tension just to get a bike on the road for a winter tank.
Now with a couple of months of salt, filth, lack of maintenance besides throwing more gunk on, and a heavy rider who likes doing the trick where you kick off and then stand one footed on the bike for a revolution or two on the chain, there's enough stretch such that I've thrown the chain three times in two days.
Today was potentially catastrophic: just going along, just gone through a green, no traffic, and hitting a bump the chain was thrown from the chainring, wrapped around the cog rotafix style, and caused me to have my first skid with low pressure knobby tires in a small gear at like 5mph on broken pavement. Unintentionally!
But all was well, there was a freshly littered snack wrapper on the ground to get the chain back out without too much gunking of gloves, and I'm alive to tell the tale of how much is too much slack.
I'm not even sure the chain tension is the culprit here, I think that my bent and tweaked fender was actually creeping sideways enough that the broken frame mount just under the zip tie touched the chain and derailed it. But if a touch can derail it, it's too slack.