Keep it cool and dry, and it will last your lifetime. A lot of old and cracked WWII 8mm ammo which was kept in warm and humid conditions still functions fine, so you shouldn't have any worries if you store it properly. What I do is I fill up 50 caliber cans with ammo, and pour some silica gel cat litter in the bottom.
By the way, 500 rounds is basically nothing. You should always have as much ammo as possible.