Certified GKH Gardening Expert
There are too many factors to give any kind of number to this answer. Generally, you will want to water for 30 minutes to an hour whenever the soil is completely dry down to about 2 or 3 inches.
This can depend on many factors, such as temperatures, humidity, rainfall amounts, and so on. The best way is to, physically, check the moisture content of the soil. This can be done by hand, or meter. A good soil moisture meter isn't expensive and a good tool to have around.
Other factors include the specific water requirements of each plant, and how much water they need in general.
This article will help you to water plants correctly in hot weather: https://www.gardeningknowhow.com/garden-how-to/info/watering-during-heat-waves.htm