Do a test and see how many days it takes without water for your plants to droop. I think you will be surprised. Only water back what the plant needs to not droop for 3 days...now you know how much to water every other day. It is going to be a very small amount in that soil mixture.
Yeah it can last a long time before drying to the point of drooping. I never let it get that dry though because in pro mix and bottle nutes you will get salt buildup and nutrient concentration that way, and will living soil the bacteria need wet or go dormant. But Ive only had to water them 2 times, they got rained on once outside as well.