If you want to water daily for 15 to 20 minutes you will need to use a Fixed Schedule.
If you research lawn watering everything tells you to water more deeply but less often to encourage root growth.
A Flex Monthly schedule calculates how much water is required and uses the historical weather averages to determine how often to water. Right now it is watering every 3 days. But as the weather warms up the frequency of watering may increase to every 2 days. At the beginning of every month the system applies the weather averages and makes any necessary changes.
How often does the schedule water in July and August?