depends on the paste, but safe to say 10 years is probably beyond the duty cycle. its $10 for a tube of arctic silver (I went with Arctic's organic, non conductive one I forget the name) that you can use for 10 or more applications. it takes 5 minutes to replace the thermal paste on a cpu.

remember intel cpus will simply throttle themselves at high temps, so replacing the paste will allow heat dissipation across all the cores (as I witnessed first hand)