I stick my finger into the pot up to the third knuckle. If it comes out clean, time to water. (like the toothpick in the brownies type of thing)
I also use 10" and 12" pots. 12" is almost 5 gallons, possibly four...not sure.
Anyway, during the summer, my ladies in the outdoor flower room in the big pots can suck-up about 1/2 to 3/4 of a gallon a day. (400w HPS, or so) I live in the desert tho, so humidity is real low. During the winter, it's more like 1/2 gallon every couple of days.
Also, if I lift the pot, and it feels freakishly light, I know I should have watered the day before, lol.
But...I'm a big fan of not overwatering, and dislike cleaning-up overflows, so rarely do I let them get that soggy. (flushes excluded, of course)