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Seems to be hardcoded, not something that can be configured.
https://discourse.gnome.org/t/how-to-reduce-system-shutdown-count-down-time/28589
But just FYI you don't have to wait the entire 60 seconds, the prompt does have a button you can click if you want to continue without waiting 60 seconds. (unless we're talking about a different shutdown/power off prompt)
Yeah, but tbh I’d rather just click shutdown & have my PC shut down instantly lol
I don't think so... I think you can disable it completely, but not change the length of the shutdown timer.
There are some extensions that you can get that can do this... or you can open the terminal and issue a shutdown command there, where you can specify a time to shutdown (or reboot).