@Ruthlilycat - As there was no internet service this is what Rachio Technical support folks had your husband do -> https://support.rachio.com/hc/en-us/articles/115010541108-What-are-Limited-Schedules-Generation-2-
One of those schedules is fifteen minutes per zone twice a day.
I think what @mckynzee is referring to is the number of different schedules that can be created and stored on the Gen 2. If there was a cycle continuously schedule created one would still need the internet to be up to kick it off. After the schedule was started the Rachio unit should keep running it even if internet connectivity was lost, until power to the Rachio (and thus the sprinklers) is lost. The advantage of a cycle every zone for X minutes schedule configured in the application is that unused zones could be skipped e.g. zone 1, 2, 3, 4 and then back to 1 again (using my installation as an example).
If internet connectivity is lost before the schedule is started, then there would need to be an additional schedule stored in the controller for use like mentioned in the article linked. However, I don't know if there could be enough smarts in the schedule to only run zones that are actually hooked up. Therefore, it could run zones 1, 2, 3, 4, 5, 6, 7 and 8 - even though in my installation only zones 1, 2, 3 and 4 are hooked up). If this is the case, then one would need to spread the zones actually in use across the controller so the "dead" time is interwoven with the other schedule instead of all at one time - as in 1, x, 3, x, 5, x, 7, x (for my four zones - move 2 to 5 and 4 to 7). A bonus feature would be it the duration could be set for each zone - otherwise, I'd say default to 1 minute per zone (open to other suggestions).
As @eunchan pointed out, this emergency setting would only work while there is power to the Rachio and thus the sprinkler valves and an additional item water pressure in system feeding the sprinkler system. While opening the bleed valve at the solenoid would help in the situation where power is lost, I don't know if there would be enough usable water pressure will all the zones in the system open at once to provide enough coverage and wetting action. Then of course there is the potential for so many homeowners setting their sprinkler systems to run continuously that the water pressure needed to fight the fires would be diminished for the firefighters (solving one problem, creates another).
While an interesting request, the ground sprinklers, while protecting grass and shrubs from catching on fire, I don't think would have prevented fireballs from landing in and catching larger trees or the roof on fire.
Glad y'all made it through and an interesting use case.