I don't wait for them to struggle with an obviously stuck shed, if they can't get it off in an hour or so I help them out.

Sterilite tub with lid, 1 to 1 1/2 inches of 85 degree water with a towel in the bottom for 30 minutes.
If by then the shed is still not coming off easily replace with more 85 degree water and allow another 20-30 minute soak.
After that soak I can about guarantee you the shed will be all off or close to it.
*I also place a half log hide in the tub for the snake to rub against*