I haven't seen/heard about any set limits with thermal pads. Usually, you change laptop because of general performance, not cooling issues. For thermal paste, the recommended switch timeframe is 2 years. However, if you don't see any change in temps, there's no actual need to change the paste. Lower end paste's have issues with drying in longer uses, but such is not issue with higher end stuff like those you mention.
The difference between high-end pastes is minimal, and usually people buy just by brand preference. For example I prefer Noctua.