Well, one think is "should", and one completely different is "must"..

Why??.. I live in a very polutioned city, and every 3-4 months I "must" take apart my entire machine, pice by piece, do a full clean, and assembly it.. It's amazing the amount of dust it can grab, despite the case filters.. Remember: dust is a mortal enemy to your system..

Of course, that involves my H80 rad, my GPU cooler, and everything else. You know, cleaning the rad fins is not easy.. In fact, I sumerge it into pressured water for a better effect (same situation with my old Tt Frio).. All the cleanning take me an entire day..
Obviously, I must take out the cooler, and as re-assembly it with the old TIM it's a fail, I put new one (CPU+GPU)..
So, despite it "should" least for several years, in my case is every time I must..
If you don't suffer for polution/dust where you live, then from 3 to 5 years would be a nice number.. Again, like almost everyone had said, if you see some temp's spikes, then maybe you'll need to do it..
Just an opinion..
Best regards..
