This is dependent on many factors including plant water use, net application rate, and the management allowable depletion.  The simplest way to decide is to use The Water Balance Method which assumes that the crop root zone is a water reservoir, similar to a bank account: As the crop uses water through the process of evapotranspiration (ET), water is withdrawn from the account.  This water can then be replaced by rainfall or irrigation deposits.  A running balance keeps track of the theoretical water level in the water reservoir, and actual field monitoring verifies the theoretical balance before final irrigation decisions are made.

At a minimum, two things must be known to successfully schedule irrigations using The Water Balance Method:

1. Theoretical crop water use (to determine daily withdrawl).
2. Irrigation system net application rate (to determine how much water is applied per hour of irrigation).

The theoretical run time can then be calculated with the following equation:
Run Time (hrs) = Crop Water Use (inches) / System Net Application Rate (inches/hr)

Sample Problem:  If crop water use is 0.25" per day, and the net application rate of the irrigation system is 0.10" per hour, how long should the system operate per day to replace daily crop water use?

Sample Solution:  Run Time (hrs) = Crop Water Use (inches) / System Net Application Rate (inches per hour)
= 0.25" / 0.10"
= 2.50 hours, or 150 minutes per day

Although, there are numerous management strategies that alter this simplistic example.