You can work it out on the basis of what you would need in cash if you suddenly because of a cyber attack could not pay by any other means, as you know what your groceries etc. bought online cost.
It would be a much larger amount than you would want to have in the house as an emergency fund, so I would suggest a quarter or en eight of what you normally spend on housekeeping every month.
The real snag here is, that if a cyber attack makes it impossible to bank online, that we will have no way of paying utility bills - the relevant firms are miles away, and probably the ATMs won't work either.
And if you do have cash in the house, make sure of two things.
The first that you do not keep it in the places a theif would check - your desk, the kitchen cupboards, your underwear drawer etc. - the police will be happy to help with a list of the places NOT to keep valuables of any kind.
The second is: make sure that the safe place you chose, is one you can remember! Seriously. We have all heard of people who put things away in a safe place and then could not remember when they need ed whatever it was, where or what the safe place was. My mother was an exponent of that art, all her life, so beware.