For water changes on my 5 gallon I put a few tablespoons of leftover stratum in the bottom of some 1 gallon water jugs. This has worked well for the small quantity of water, so far the jugs have gone through about 2 months of use with tap water and the buffering capacity is still holding in them. (somewhere around 8-12 gallons of water, I forgot to keep count lol)
From others posts I've read on various forums people say you can probably expect around 6-12 months of buffering from the substrate using straight tap water. (ultimately depends on how much KH is in your tap water) Using water already stripped of the KH should give you 1-2 years.