In theory sustained heat is not ideal but - for space reasons I have had to store some bulk ammo in my old shed - and that is mega hot most of summer.
Some Wolf and 8mm milsurp was used after two years with no apparent problems. In fact even some old .303 Brit and .308 come to think of it also had to live out there. No failures I can attribute to heat.
If possible tho - store at room temp or cooler, and in the dry - as a more guaranteed method to ensure no degradation.