For me it's weeks to months, but to be honest I mostly do what's convenient however long that gives me. I don't lie awake at night worrying about it.

For example, I have a stockpile of tinned carrots and tinned sweetcorn, and I have 3 times as much of the latter as the former because the local supermarket sells it in packs of 3 which are more convenient for storing and rotating. I have a certain amount of space available and it all gets filled up. I keep less flour now than I used to, partly because I found I wasn't using it quickly enough and it wasn't producing very good bread. I keep roughly a year's supply of coal (at normal usage rates) because that's how much space I have to stack it in.

I do think that 2 weeks is a good minimum to aim for. As I live alone and have no dependants (other than a cat, who frankly can fend for herself if it ever comes to it), it's an easy target to hit. I think I have food for around 3 months, but realistically I expect to be sharing with less prepared neighbours so I've no idea how long it will really last. No plan survives first contact with the enemy.