Stock up for a maximum amount of time as 6 months. You don't want to keep rodents for longer than that.
Make sure the freezer bags are properly sealed at all times, if not it will cut down the life of the frozen prey item drastically.
5 snakes X 4 feed per month X 6 months = 120 rats maximum.
You can fit that into a sweater box rubbermaid.
So use that for size comparison.
-Steven