Once, when you install the CPU.
I agree. You will often see folks say that TIM (thermal interface material) wears or dries out. That is not true as long as
the cured bond between the two mating surfaces is not broken. If the bond remains intact, the TIM will last indefinitely. It is more likely the fan bearings will wear out first.
But if the cured bond is broken, then you must clean the surfaces and apply a fresh new layer of TIM. Note that heavy heatsink fan assemblies can break the bond if the computer is rough handled or bounced around during transport.
I disagree with the "blanket statement" that SpeedFan is no good. I will say that it does not work correctly on some
systems, but I will also say the same thing for HWMonitor and most other monitoring programs too. These programs often put the wrong label to sensor so you may see the System Temp when it is actually the CPU Temp. Your motherboard utilities disk should have a monitoring program (or check for a more recent version on your motherboard or PC maker's website). If none, I recommend CoreTemp
for newer Intel and AMD64 CPUs, or RealTemp
for Intels. These programs too may not correctly label a sensor so I use Everest
to verify the temperatures (as it is usually able to match sensor with label correctly), then edit the label in the monitoring program. In Everest, look under Computer > Sensor, then wait a couple seconds for the readings to appear. Unfortunately, Everest does not minimize to the system tray to show real-time temperatures, otherwise, you could use Everest instead of the others.
Do not rely on the temps shown in the BIOS. While they are likely correct, running the BIOS Setup Menu is probably the least demanding task you can ask of your computer so it does not show the temps when the system is being taxed.