Usually 28-29 days, but temperature can play a part in this. I have found that diamonds kept outdoors can go as long as 32 days if the days/nights are cold and they just cant be bothered to emerge to get it off(even when the skin was ready to come off on the 28th day). However, if the days/nights were quite warm they will emerge at 28 days to shed.
I'd imagine this to be true of all python species, they're willing to prolong the days until their prelay shed if weather is a deterrent