Apple says the user is notified "once a day", and updates are forced once deferrals are used up. The deferrals do not decrement if the device is powered off. The count starts from the date the device received the command, not the date you sent the command.
MaxUserDeferrals integer The maximum number of times the system allows the user to postpone an update before it’s installed. The system prompts the user once a day. This key is only supported when InstallAction is InstallLater and only supported for minor OS updates (for example, macOS 12.x to 12.y).