the duration will be determined by the zone efficiency, root depth, awc and the pr.
i would say your root depth is reasonable for st. augustine, probably ok for bermuda if you are in sandy soil (but probably not with that awc).
im assuming you have used to soil database to reach your available water capacity. (http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx)
the wild card here is the pr of your heads. i think this gets overlooked or ppl take a wild guess. the best way to determine this is conducting a catch cup test. if you want to spit ball it, you can look up your heads on the manufacture’s site and get an idea, but you will need to know your water pressure. if you like to live dangerously, take the middle of the road in water pressure.
once you get the pr set correctly, the flex schedule will compute the duration of your schedule, its really nice, everything i have runs on flex.
please feel free to ask more specific questions about any of this, its hard stuff but in my opinion worth it.