This is a simple tip. Go to the terminal and type:
sudo shutdown -h +45
After that your system will shut down in 45 minutes.
You can also specify the exact time:
sudo shutdown -h 22:30
If you want to cancel the shutdown timer you should use the flag "-c":
sudo shutdown -c
You can also use the flag "-r" or "-s", to reboot or make the system sleep.
man shutdown for further documentation.