They can die within six hours if they don't clean-I've had it happen a few times-we usually give them oxytocin every hour for six hours and they will pass it most times. I was real lucky this year a mare had all but a little bit out we treated her and she took a bit longer than was safe to get rid of it. The neighbors mare got so fevered from not cleaning she laid in the slough and died-another died in the vet clinic. It's definately not at all like dealing with a cow.