Sleeptimer is a zenity bash script for automatic shutdown your system quite similar to Kshutdown and Gshutdown mentioned in this post, but with fewer features just for halting the system. The idea of this script inspired from automatic shutdown the system after watching movies without bothering yourself with closing currently active applications and windows, it will halt everything.
Sleeptimer required zenity and bash.
Download Sleeptimer from Here.
Simply execute sleeptimer script and enter the required time till shutdown the system by minutes.