how long can crumbles last on the ground ?

I finally had to switch to pellets due to the same problem. My birds will not eat the crumble if it sit out overnight. I do have a one that hates the pellets and one that chokes on them (she is tiny) so I grind up a little for them. Perhaps you could try switching to pellets and only put down what they will eat in one feeding.

how much food can you leave on the ground before it becomes a health hazard

If it gets wet it can start to mold almost immediately.

If there is already that much build up, you're refilling the feeder too often.

Clean up what's there now, and then don't refill the feeder until they clean up themselves

It is unsafe for them to eat food that is moldy. Stale is ok, but not moldy. So I recommend a catch pan if possible or some other change in the way the feeder is set up. I personally would excavate that ground and bury all that old food so they don't get to eat it. Then the fresh dirt will be on top.

I would definitely switch to pellets as soon as you are through with what you have now.  We don't let our 12 keep feed on their feeder for more than a day or so, I would get rid of that old food.

I also have a similar problem.  I have 3 pullets that are too young to join the flock, and they throw more food than they eat.  It builds up under the cage and the birds in the run scratch around and eat it.  Guess I'll be cleaning it up this evening after work.

Ian

Never heard that before and have never had a problem with it at all.