My grow:
promix hp no amendments, 5 gal fabric pots,
general hydroponics flora trio plus
calmag.
There's 3 things i look for in my grow to get an idea of when they need a drink.
1. I check my hydrometer often to make an average of my rh/temp before and after feedings. My rh stays in 60% to 70% range around 72 to 74 degrees after feeding. When my rh drops to the 50s % I know they are getting close to being dry. Usually I have a day or two at most after that.
2. Feel the trunk of the plant a few inches above the soil. If it feels kinda cool to the touch then it's still drinking fluids. If it feels room temp or you're unable to tell difference in trunk temp then it's close to needing a drink. At this point I have 1 day at most before they're too dry.
3. Feel the weight of the pot. If it feels light like close to what it felt like when u added the soil to the pot when you started ther plant, she's about ready for a drink. Got about a day at that point.
Sometimes if I'm in a rush I'll even stick my finger in the soil about an inch or so to feel if it's still moist or dry. This is more to see how dry the top layer of soil is atleast. Usually top of soil is gonna be the driest but if you can feel moisture an inch down it's probly good for a couple days usually for me.
This is just from my experience and techniques.