It is common with all TIMs to decrease in efficiency a few degrees over time but it should never be enough that it puts you dangerously close to thermal thresholds. That is, if you "need" those extra few degrees to keep from throttling back in speeds or reaching some other thermal protection threshold, you have other more important cooling issues to deal with, like a broken bond on your heatsink or inadequate case cooling. The point is, TIM never needs to be replaced because it is x number of years old AS LONG AS the bond between the CPU and heatsink is not broken.
Remember, there is NOT ONE, not a single solitary TIM maker, cooler maker, processor maker, motherboard maker, graphics card maker, or computer maker that says the TIM needs to be replaced on any regular or scheduled bases. Not one!
If it was needed, at least the TIM makers would recommend regular replacement as that would significantly increase their sales profits.