I prefer to NOT take any chances with my $2500 dollar computer so I replace the TIM paste at intervals not to exceed 1 year...but that's just me being extremely cautious.
A year to two years is a good time table...this removes the possibility of the paste drying out and causing catastrophic failure....however with today's built in thermal protection circuitry designed to prevent castastrophic system failures...you would "probably" not ruin the cpu...but why chance it...I change the paste on the cpu and the gpu...AFTER THE WARRANTY RUNS OUT...at reduced intervals of 1 year.
Best practices says from 1 to 5 years is alright...most of us don't keep the same system for 5 years but hey mileage varies among all of us geekers!!!
Respectfully,
tekman42