Under normal circumstances, thermal compound or paste should last at least a year once applied to your CPU (or GPU if installing something like, say, an Accelero Xtreme IV cooler), and potentially much longer (as in several years), depending on the type. However, that might not be the case if you picked up a tube of Arctic’s popular MX-5.

Source: Hot Hardware – Seeing High CPU Temps? You May Have A Bad Batch Of Arctic MX-5 Thermal Compound