How much should you water a garden?

The short answer: As often as your plants need it.

Watering requirements will depend upon several factors: the soil condition (and its ability to retain moisture,) weather (amount and frequency of rainfall,) and the water requirements of your particular crop. The best way to tell is to know your crop and the indications each particular plant will give if they are thirsty or over-watered (hence,t he short answer.) Additionally, in the U.S., the USDA has Cooperative Extension offices in each state. These offices provide (free) information on typical soil conditions in your particular area with additional information on each specific plant based on these soil conditions and the climate. See the link below to find one nearest you.