I use them and am also curious as to how long most people use them. Mine are on the lowest temperature and only in one corner so that they can easily move if they get too warm. Sometime around 3-4 weeks they start to be able to regulate their own body temps better and it won't be necc., I am still using mine and they will be three weeks tomorrow - they still love to nestle up in that corner. It is just slightly warmer than the rest of the box.
Don't know if this helps at all.
Being that it is still winter I will probably use them until they start staying out of that corner.
By the way the pads are under the box and the floor of the box is covered.