They
should last for years
if used correctly. This means only reheating (as per instructions) once they have fully cooled. This is because they are made using a phase change substance that alters its state when heated/cooled.
If you reheat it before it has cooled you put too much energy into it and it will melt the container it is in and die. Used correctly they should work for years. Mine is 4 years old
I would personally advise having 2 or 3 and just rotate them to avoid the temptation to reheat them too soon. :wave: