i have 5x MX-4 and 2x MX-2 thermal paste. All 4g each. For one CPU?
if you are OCD, you can swap thermal paste every week...then those 7 tubes of 4 gram won't last that long
but if you are like any typical enthusiast....every 6 month to a year is always a nice range....and a few tubes will last FOREVER if you apply paste every 6-12 month