Personally it depends on a couple things when I'm watering:
The size of the plant and how dry the medium it's growing in would be a huge factor, as the smaller plants probably aren't gonna soak up the same amount as the huge ones, so I sometimes do a "finger inspection" (Rub on the top soil and see how far down it goes with dry).... Could say finger it until you feel a wet spot but I don't want to make this be a mind in the gutter post
LOL.
Another thing that sometimes comes into play with that are genetics; Motor Breath for example doesn't seem to particularly like heavy waterings until it's huge; Damn near killed the first two clones of that I had the first time I grew it because I over watered them. This year I have Tashkurgan and it's a pure landrace heirloom indica, and I'm terrified of watering it too much given that where it came from I'm thinking "dry" for the climate.
Normally when I add water to my pots, I watch how fast the soil takes it in. If the water goes in and it's just gone, I give some extra. If I put water in and it sits there for a bit before being absorbed, then I have to gauge if that was because it's already wet (Finger checking first would help) or if the plant is wilting because it's in need, that at times will also have water sitting on top for a moment so generally I eyeball that.
I also have a decent sized Sin City Juice plant that will slam water down like..... Seriously she drinks more than I do LOL. When it's hot outside, she starts to wilt like every time, and I end up dumping almost 2 gallons of water in her pot, which is quickly absorbed too.
Are you setting it up so that it drips out over the course of some time ? I don't like "runoff" myself personally; I figure the soil and the nutrients I add cost a decent amount of money, and I want that water to stay in the pot even if it means watering slowly so nothing spills over the sides, but it can be anything from a half gallon of water a day, or less for smaller plants, up to multiple gallons being added in to a particularly large plant (I have a GMO that's massive right now and will slam down 2 gallons a day no problem)... I hope at least some part of this post will help you because I'm having issues trying to describe what I do. And a timer seems like it'd be a great option if you can balance the flow just right.
TLDR:
Trial and Error For the win.
She is super easy going and cruising picking up some interesting sweet chemmy notes on the stem rub.
