Create a new ringing style - Style 2 in this example. Set the phase 2 delay to the required duration that the sounder will ring for -- 1200 secs = 20 minutes in this example.
Select OFF in the output type, this will then ring the sounder for a 20 minutes duration and then turn off.

In the Cause-and-Effect groups select the sounder group 198 (this if the default for sounders). Ensure all wireless sounders are in this group.

Set the ring style 2 -- this will then pull through the 20 minutes duration on all the sounders.