How often should we replace thermal paste on CPUs and GPUs?

I call that response the "don't fix it if it is not broken" solution.
you replace it when its not doing its job anymore i.e when your cpu is getting hot and there is no specific reason like dust or fans not working to cause the heat, thats when you replace.
Or if you remove the cpu cooler then you have to replace the paste as the old paste is now redudant.
there isn't a life span as far as I know, as I said when its not doing its job you replace it.
Paste gets dry after a while but thats inevitable but it doesn't need replaced unless its not doing its job.

They should be replaced only if thermal module or MB disassembled.
Otherwise, if the laptop start to heat, it is recommanded to replace it and test before replacing all the thermal module.

At most once in two years

you better do it only when needed