Q: When should you do that.
A: The time when you need to re-apply the thermal compound is usually after around 2 years. The reason for this is because after time the ventilation ducts start to become blocked and cause the laptop to heat up more than normal thus putting more stress on the themal compound, don't get me wrong - the paste is designed to withstand and/over the temperatures of the processor but for example if you have a toasty P4 installed and its been running and getting thrashed for about 3 years its not going to be at its best. To find out you can download a free program from cpuz.com called PC Wizard 2008 and this will tell you the temp of the cpu, I prefer Everest Ultimate 2008 but its personal and prefessional choice. If your laptop has shutdown unexpectedly when the fans were going crazy that is also another let tale sign of overheating. another example it to have to laptop idle for 10 mins and if its over 55C degrees chances are it needs the ducts brushing out and new thermal putting on. If you decide to replace it, buy a proper thermal compound with silver particles because they increase conductivity, beware though as this type of compound is electrically conductive so only put it onto the heat transfer plate and be very tidy. You need to purchase a proper remover, something like TIM cleaner which is perfect, it is a citric based acid and is easy to use, clean both face plates, leave no grease!!!