I always have 30 to 40 gallons on hand. We have those office-cooler type 5 gallon bottles delivered every two weeks. We keep 6 to 8 of those bottles in a queue, drink the oldest ones first, add the new ones to the end of the queue. You don't even have to think about it. They deliver the same number of full bottles as you put out in empties. So if you drink a lot and put out many bottles, they will delivery many bottles. Likewise, if you go on vacation and only use one and put that single bottle out, that's all they deliver. The exception being if you don't put out any bottles (forget, or whatever). Then they deliver two, or whatever the default is that you set up for your account. Although having the bottles delivered is slightly more costly than you going and buying them at the grocery store yourself, it's worth it to me to have a "no thinking" method of water management.

p.s. - I don't know exactly how long 40 gallons would last the two of us in our household, but it should go pretty far if we conserve.