Noctua NT-H1 and NT-H2 thermal pastes have a specified usage time (on the CPU) of up to 5 years. However, you will most likely see excellent results for much longer than that, so we simply recommend checking the temperatures occasionally after passing the 5-year mark. Unless you see a clear increase in delta temperatures (difference between CPU temperature and ambient temperature, compared at 100% CPU load) there is no need to replace the paste. Many customers have been using NT-H1 for 10 years or more with excellent results.