SNAP is a USDA program to supplement the amount of food poor people would be able to buy on their own. But how big is the supplement? The US military pays out funds that are supposed to cover all meals, whether provided by the government or not. This is the BAS or Basic Allowance for Subsistence. Officers and enlisted get different numbers.
The 2014 maximum SNAP grant for a 1 person household is $189. The monthly 2014 BAS for an officer is $246.24 and for an enlisted rank is $357.55. So it's a reasonable equivalence to say that between half (comparing it to enlisted BAS) and three quarters (comparing it to officer BAS) of what the US Government believes you can survive on and function for food is available via SNAP.
This puts the "foodstamp challenges" you can find across the Internet in something of a different light. In these challenges, people are asked to fit their entire food budget into the SNAP limits. What's scary is that sometimes people actually pull off a successful challenge anyway.