When I lived in the US it was ~100 bucks cash and my amex and my visa. Never carried my ATM card cause that sucks if you lose it and you don't get rewards and basically there is no reason to use an ATM card over a credit card.
But my point is, I'd carry like 100-120 on my and spend it the rare places where I couldn't use cards (ie drug dealers and older friends that can buy booze), then when I was down to about 40 i'd go get more. I once went like 5 months with out having to go to the ATM. Cards everywhere.