There are many factors that determine how much water a plant needs such as the weather, soil condition, and the type of plant.  Try to keep the moister content somewhere between 40% and 80%, enough so that soil can hold together to form a ball.

To test soil moister level, take a sample of the soil.  The best tool is a soil sample probe although a trowel can also be used.  About 6” of soil is usually adequate for gardens.  Hold the soil sample in your hand and squeeze it.  It should hold together with defined finger marks.  If it falls apart easily, you may need more water.  If it sticks to your hand or water can be squeezed out, your soil is too wet.

Soil moister meters are also useful but it is important to get one that can be calibrated.  Set the high level when your soil is completely saturated.  It is usually time to water when the probe shows less than half way.  Do not leave the probe in the ground.

A quick method is to put a small rod or screwdriver into the ground.  It should go in easily and have some material attached to it when you pull it out.  If it is difficult to insert the rod into the soil, it is likely too dry.  If a lot of material is attached to the rod, it is probably too wet.