It all just varies. The system is set on a certain number so it goes off sale depending on other customers orders, etc.
A code put in that says Grant is on sale until there is only 200 left.
So if there are 250 kits and 50 people buy one, it goes off sale. If 49 were purchased and you put one in your cart but didn’t pay yet, some one else buys one in the mean time, it goes off sale.
So if I am ordering one I know wont stay on sale long (newest realborns, etc) I grab it fast and check out.