I have not looged on here for a while, but just did to give you guys a method I used and worked for me, since I use OMV on my laptop and wanted my screen to go off after 60 seconds. So here we go, hopefully it will work for you as well.
1. Log in using putty to your OMV. Use root as a user.
2. Enter password.
3. Type nano /etc/default/grub
4. Find GRUB_CMDLINE_LINUX and add "consoleblank=60".
60 is amount of seconds after which screen will go off. Change it to your liking.
5. Now press CTRL-X, click Y, then Enter.
6. Type update-grub.
Restart OMV, screen should turn off.