There’s no real specific rule, just industry standards, or at least what the big companies do.

Roughly, sample stored at 45C for 12 weeks = 1 year stability at Room Temperature. 

This is vaguely based on the Arrhenius equation which predicts chemical reaction rates of systems. Reaction rates double every 10C.

So if RT = 25C and lasts 1 year
the same system at 35 C would last 6 months
the same system at 45 C would last 3 months

Of course, this is just a rough guideline because you don’t really know what would be causing something to become unstable. If it is microbial contamination then these numbers wouldn’t really hold up.  But it’s a rough guess.

I think the guideline for longer stability is 1 year at 45C = 3 year RT.  However, big companies don’t expect cosmetic products to last more than 1 year.

See this post on cosmetic stability testing.