The weather forecast says hot and sunny for the next week, with no rain. According to conventional wisdom, a vegetable garden needs an inch of rain a week. A good, soaking rain will provide 1/4" or more, so a few days a week sufficiently waters a garden.
Without rain, a vegetable garden needs supplemental watering. In sunny weather particularly, the soil will rapidly dry out. First, do the finger test: poke a finger down to the second knuckle. If it comes out dry, the soil needs watering.
Second, consider the maturity of the plants. Young seedlings do not yet have deep roots, and so depend on moisture at the top. They need watering more frequenty -- at least every other day. Mature plants ought to have deeper roots, so as to need watering less frequently. However, unless they get some un-watered days, they don't have incentive to root deeply. Plants adapt to the watering they are given, so watering every day means you'll have to continue watering every day.
For our garden mid-season, we have a mix of maturing plants and seedlings. This comes from rotating crops -- for example, when the peas came out Saturday, the pole beans went in -- and from staggering plantings, so that we get multiple generations of things like carrots and lettuce. So this forces us, in the absence of rain, to water at least every other day.
We prefer to water in the morning, to reduce loss to evaporation in the hotter and sunnier part of the day, and so that the leaves don't stay wet overnight and acquire mildew. We also prefer to water by hand, with watering cans, to put the water where we want it, and not just throw it up in the air with a sprinkler.
How many watering cans-full do we need to apply? Just how much water does 1-inch a week mean?
A standard bed in our garden is 6' x 9'. 54 sqft x 1-inch = 4.5 cubic feet of water per bed. 1 cubic foot = 7.5 gallons. Thus, our beds each need 4.5 x 7.5 = 33.75 gallons of water per week. With 1.5 gallon watering cans, that's around 22-23 full watering cans. That means, when watering, we should water each bed with at least 3 full watering cans -- and probably more, since we don't want to water every day.
Our entire garden is 2000 sqft, with about 60% for growing plants (much of the remainder is used for paths for visitors as well as ourselves). 1200 sqft x 1-inch x 7.5 gallons/cuft = 750 gallons = 500 full watering cans. Watering five times a week means 100 full watering cans each.
This sounds like a lot of water! But compare to how much water you use to shower yourself. A typical modern showerhead has a flow of 2.5 gallons per minute. So the water we need in our garden per week is about the same as showering for 300 minutes. How many showers do you take per week?