I’ve been there, only eating what was in our pantry. I thought we had 2-3 months in the pantry but it was empty after a month.
So now I don’t guestimate in my planning but count actual meals and calories.
I count calories and have a tally sheet showing how many 2,000 calorie days of food are stored. It's enough to carry our family rather well, even without supplementing with squirrels, birds, gardening, and other sources. A family of four should need just over 1400 man-days of food to last a year.