When is the right time apply thermal paste again?

[Spartan191]

Okay so about 6 months ago give or take I upgraded the the stock cooler on my FX-8350 to a 212 EVO. It came with some nice thermal paste and ever since I installed it the temps have stayed pretty even. I even did a little overclocking and it's running stable at 4.5GHz. So I figure what gives if everything is running stable. I thought I'd ask just to make sure though.

 

[rezeile008]

whenever you remove the cooler from the processor