You can purchase a laptop tray, to make sure that your clothing does not block any of the cooling ports, which are located underneath most laptops. Then it is OK to work as long as you like.
Some laptops will get very hot quite quickly if you place them directly on your knees as clothing tends to get sucked into the vents and then blocks the air needed to cool the processors. The design of the vents assumes that the laptop will be on a hard flat surface such as a desk or counter top.
The original term for a laptop was portable computer, and at that point we had not envisioned the day when they would be so small and light that you could literally place them on your lap.