One important thing to note is the shelf life on MRE's vary both by type and storage conditions. Anyone new to this idea needs to do their research.
There are two problems for preparation. We are typically restricted in the quantity we can buy and they often don't have a long shelf life.
I myself have enough MREs for about one month. I get mine from mealkitsupply.com and hotpackmeals.co.uk. These are self heating ration packs which is nice if the electric cuts out.
The mealkits last for five years, probably ten years to be honest. I've seen people eat MREs from the Korean War and they were perfectly good to eat.
The hotpacks last only three years, but are quite nice to eat and I tend to use them up as I go for lunches.