i cant give you a finite number of gallons. i can tell you that i saved ~$200 last year based on run times and my pricing tiers with the county. with flex schedules enabled, i ended up skipping a lot of waterings because it was a pretty wet year for us last year and my rain sensor was dry within 2 days after the rain, where flex was typically taking me out 5 days after the rain before watering.
that is just a rough comparison on when my old system should have run compared to iro with flex schedules actual runs. there could be some error in my method.
anecdotal, you will notice it, even my wife whom could care less about irrigation was noticing the difference since the push notifications were coming to her phone. and even she noticed the system was running a lot less than our old pro c on a fixed schedule.