Hello @Annissa Boyce ,
Thank you for posting here.
For your request "Warning to the user that the system will reboot ..eg 15min".
I have done a test in my lab.
If I configure User Configuration\Preferences\Control Panel Settings\Scheduled Tasks\right click Scheduled Tasks\New\Scheduled Tasks(at least Windows 7), it will prompt user.
1.Create an OU and put domain user accounts into this OU.
2.Create a GPO and link this GPO to OU above.
3.Edit the GPO as below.
In my case, it it 20 seconds (you can set 900 seconds).
4.Logon one domain client using one account in the OU above.
5.When the machine update the GPO and it will prompt the user in 20 seconds when the machine will restart.
Hope the information above is helpful.
Should you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou