Water storage emergency backup

How many days’ worth of water storage, in case of disruption, is reasonable? 

If I have a pump between a well and a rain barrel and an additional pump between the barrel and the household, how many days’ worth of extra storage should I have? If the well gets contaminated, disinfection could just take a day, but if a pump or some other component breaks the delivery time could be around a week for example.

What’s your take on this? I’d also be very interested to hear what your setup is in terms of water supply.