Hello; Lets go back to some of the information already posted in this thread. The powered form of SAFE is stable for about 30 days after first mixed. Why not estimate how many water changes you plan to do in a 30 day peroid and try to get an idea of the total amount of water used. You could then mix the amount needed to treat that amount of water. Label the container with the date mixed and how much water it should treat in total. Divide that by the amount of water used in each change.
Example could be that if you change 50 gallons each week, then you will need enough to treat around 200 gallons. ( 4 X 50 gallons) This may make measuring out the doses a bit less tedious.
On another note, have you checked a water quality report from you local water company? Some, myself included, are fortunate in that our tap water is treated only with chlorine. I can and do store my tap water for a few days (usually at least four days) and then it is fine to use in my tanks. I have some SAFE and PRIME on hand to use under unusual conditions such as when I clean equipment with clorox solution.
On the other hand many have their tap water treated with chemicals such as chloramine. If your tap water has this stuff in it then the general consensus is that you must use some sort of water conditioner like SAFE or PRIME.
If you are very lucky and happen to be on a private well, then you may have the best situatioin of all. I had a good well for a number of years and all I needed to think about was the temperature differences during a water change. Of course a private well should have the water quality tested from time to time.