According to Mountain House:
How to Store Mountain House Freeze-Dried Meals
Store Mountain House food under dry, sanitary conditions, without prolonged exposure to extreme temperatures and avoiding any environment that could cause damage to the packaging, such as punctures, dents, or rust.
To maximize shelf life, store food unopened and avoid prolonged exposure to temperatures above 75° (24°C). Simply follow these storage tips and you’ll have emergency food when you need it.
Uh oh...have been storing mine in the garage (gets pretty hot in the summer).
I usually use them within 12 months, normally.
The garage, the attic, the crawlspace, none of these are great places to store any outdoor gear. Summer temperatures can be extremely hard on sleeping bags, and tents in particular.
That's true for storage of anything. Cool and dry for anything you want to store.