Plants should be watered when they need it.
How do you know when a plant needs water? An “average” plant needs an inch of water a week. Of course, some plants can get by on much less, while others need to be constantly wet. Knowing the plants in your garden and the different site conditions they require will tell you how much water they need each week.
But that is not how much you should water them. If it has rained recently – or if it’s expected to rain soon – subtract the amount of rainfall from the amount of supplemental water you give your plants. Your local weather forecast may tell you how much rain your area has received lately. Or, you can set an empty tin can in the ground as a simple rain gauge. (You can also use the can to track how much supplemental water you’ve given nearby plants.)
If your plants are watered automatically by a timer system, be sure to turn the system off when the plants have already received enough water from rainfall! Too much water can harm a plant, just the same as too little.