Hello Red-Eye and ****-Dog-
I too use 5 gallon containers. I water a little differently when in veg vs. when in flower.
In veg, IMHO, the plants need more humidity, so I water more often. I'll usually stick my finger in the soil- if it feels dry about 1-2 inches down, I give them water. They usually take about a half gallon and get watered every 3 - 4 days. I try to maintain rh above 50%.
When in flower- I let them dry out more. I actually wait until they start to lose some of their turgidity- as HippyinEngland and Maineharvest mentioned (turgor pressure is one of the effects of transpiration- basically makes the leaves stand erect). So I wait until they start to droop or even wilt- but I don't let the stems droop. I usually give them about 3/4 to a gallon of water and they usually go about 7 - 8 days between watering. I try to keep the rh below 40%.
One of the things I think is important, as aplasia said, is to water slowly. If you pour it all in at one shot, it'll just run right through your container and out the bottom. If you water slowly, you'll start to see run-off at the bottom of your container. They should be well watered at that point.
As Maineharvest said, you'll get used to it and it'll get much easier.
Hope this helps. Good luck and keep growing!