A bottle of pH down is not expensive and will last a long time. Less than half a teaspoon will knock a gallon of 7.5pH down to 6.5pH. Just a WAG but I'd bet a quart will grow at least 30 plants or so if your starting water is around 7.5. Unless you have really high pH water or are growing a bunch of plants, one quart will likely last a couple of years.
I use about a teaspoon to water this whole tent at the moment, taking 7.5 down to 6.5. I could knock down my 7.5 tap water and water them at this stage at least 175 times with 1 quart of pH down.