I really don't think there can be a very hard and fast number to this. Everyone is going to vary a little in what they need, could use, and can have on hand.
If you are young, just getting started in life, and can barely scrape together a few thousand in savings right this minute, then planning to have 90% of that as cash in your hand might not be the smoothest idea. Its not exactly insured in most cases, so if someone robs you blind or your place goes up in smoke, then you're left in a tricky spot.
The nature of the community you're in, and the spirit of the people you deal with will also impact that amount. How willing are people to let things slide a day or two? How trusting are they? Can you walk into a local store, sign your name at the bottom of a slip, and have no one bat an eye at the idea that you'll be back at the end of the week to settle up?
And don't forget the ultimate in important issues: If you can't use your plastic, are you even going to be able to use cash for it?
Last summer I was up at the mall, passed the kiosk for my internet provider, and remembered that I was going to have to pay my bill sometime that week. I had the paper bill in hand, full payment in cash right down to proper change, but their power was out. They couldn't do anything for me and were waiting for someone to fix it.
I've gone to pay for meals after eating when the power went out, and gotten rather blank stares from the young staff. Not a clue how to properly calculate tax on it, no confidence to get the math right in their head, and no way to open the till so they had a reasonably secure way to store the cash me and the other customers were going to give them. Not to mention the several places with a fully automated 'print on demand' billing system: Power outage means they didn't even have a clue what my order was anymore, let alone how much it was suppose to cost me.
Originally Posted by Lrrpie-CT
I destroy drives for clients all the time. Sometimes, intentionally.