The goal behind flex scheduling is deep, infrequent (as infrequent as possible) watering to promote healthy roots and plant drought tolerance among other things.
Each zone has its own moisture details and runs at its own cadence. Think of the flex schedule just as a container for zones that groups zones together by when they should start (start time). If two zones have the same characteristics, and are irrigated at the same frequency, they will always run on the same day.
Flexible monthly is a good option and being predictable (monthly), easy to understand. A drawback and where it can fall short is that the intervals are fixed for the month. If there are significant rain events, or the weather cools down a bit, you won't see the efficiencies that flex daily provides.
I can't answer this question without doing a deep analysis of your yard, overall monthly/yearly watering etc. I do know that just with using our weather intelligence features (rain delay, freeze delay, climate skip, seasonal shift) with a fixed schedule you can save about 15%-20% off the top by doing simple automated weather skips. If you use the flexible monthly you can save even more by using automated, adjusted intervals each month. Using flexible daily I believe you can save up to 50%. In August I watered 5 times compared to the 10-15 times that I would have based on setting a 2 or 3 day interval for our second hottest month.
If you want an in-depth breakdown of my experience last year this article tries to explain my trials and tribulations.
I know there are a lot of new concepts and information to consume. My recommendation is usually to start with one zone, get it dialed in, and then transition other zones when comfortable. The reason you see a very long run time for trees is that we assume they have very long roots (24 inches), and will typically only water them every few weeks.
You can always adjust the duration down for grass if you feel that it is watering too much. If we don't get accurate inputs (i.e. nozzle in/hr, soil type) then our industry standard watering algorithms won't be as spot on. The best way to visualize frequency is using the soil moisture graph on each zone. If you feel that the frequency needs to be adjusted we also have ways to fine tune. With flex daily, there is usually some up-front work on getting it configured, but then it truly becomes set/forget with huge efficiencies. The community can typically get your system largely tuned by seeing a soil moisture screen shot and your zone settings.
One last thing, we are rebuilding flex daily and monthly schedules to make them easier to adjust without going into advanced settings, in our next major software release.
Hope this helps