A sleep timer was something I missed when I got my G-Box Q. The issue is not the G-Box but XBMC on Android. Thankfully I found this great add-on that will allow you to set a sleep timer.
After you download that file watch the YouTube video for directions on how to install and set it up.
If you have any questions please leave them in the comments we will do our best to answer them.
If you would like to buy a G-Box Q you can find it on Amazon here: