In most cases, you shouldn't need to reapply more than once every few years, though you should replace your paste if you remove your cooler for any reason. You may also want to consider reapplying thermal paste if you find your CPU temperatures are climbing.

Do you need thermal glue for CPU?

Thermal paste, or some oily thermal interface material, is necessary because it fills in the microscopic imperfections that otherwise trap air particles between the CPU and the heatsink, preventing the CPU from properly cooling.

What happens if you don't put thermal paste on CPU?

What happens if you don't have thermal paste? Things don't work as efficiently as they should. Your CPU's operating temperature will be higher. It may need to slow itself down (thermal throttling) to stop from overheating and failing.