Stash Cash for the Short Ters?

If the US Debt Bomb goes off, and banks across America collapse like a house of cards, there's not going to be any such thing as going to an ATM or cashing a paycheck. So my question is this: since the day-to-day economy (e.g. groceries) isn't going to convert overnight into a precious metals & barter based system, how much cash do y'all think it makes sense to stuff under the mattress?