Hi Twizzle have you lifted it and turned it upside down as a stack? If so a good year in my experience but we leave and lawn turf we acquire from neighbours in the stack until we need loam.

Anywhere from 12 months to 5 years, depending on where the stack is, how much water it gets and many other factors. I tend to wait until the mound is wet and then cover it with plastic sheeting, makes it work quicker.