The watering duration does not change in flex, just the frequency. Watering will be skipped if the IRO predicts the moisture balance will stay above 0% (the allowed depletion), before the next available watering date . In the graph above from azdavidr, the system waters 0.42 in, the difference between field capacity and the allowed depletion level. On May 9, it watered 0.42 in, but there was 0.23 in of moisture loss, so it had to water again on May 10 to avoid hitting 0% moisture level again. After 2 waterings, the ET was less the next two days so no watering was needed. The zone would water again on May 13 unless the predicted precip was greater than the predicted ET, as the moisture level was almost 0 after May 12.