Cool and dry place for storage makes ammunition last for about 100 years if it is 1970s non corrosive or manufactured more recently.
Those temperatures will degrade the ammo over time. Humidity is worse than heat but storing at 90 to 99 Fahrenheit will slowly degrade it. I imagine it will likely take over ten to twenty-five years. Store your ammo at 75 degrees Fahrenheit or cooler with no humidity and it will last for one hundred years or longer if it is non corrosive 1970s or manufactured more recently.I store mine in "sealed" ammo cans in hot ( 90's F ) garage. Military ammo has been stored in warehouses in hot deserts and jungles for extended periods.