I never make more than a gallon of starsan and that lasts me forever. it is a contact sanitizer meaning you can put a couple of cups in your better bottle and swirl it around then dump it back into the gallon jug.
I'm kinda in the middle on starsan usage. I buy a gallon of distilled water, put 6mL of starsan in it, and shake for a bit. I fill up a large spray bottle from dollar store with that solution, and will fill s small tub with it that I can drop airlocks and bungs into. I rack into/out ta that tub to sanitize auto siphon and tubing on the inside. Buckets I will just spray the inside down and swirl and discard. Starsan never goes back into the gallon jug, because I Dont want the trouble of testing pH. One gallon jug will last me well over a month, even if I brew every weekend.