Check marketplace etc if your in/near legal states, deals can be found. Same with non legal, there just won’t be as many.
I have 2 autopot setups, both 3.9 gallon with 12 gal reservoirs. With 4 per tent I fill each res once a week… more in stretch. With 6 to a 12 gallon res I’d expect 4 to 5 days for larger plants.
I mix in a 20 gallon Rubbermaid tote, takes 3 to 4 days for ph to stabilize which I do before filling reservoirs. Less ph drift to the plants and they run happier that way.