I'm not a big fan of using a timer to meet minimum practice time requirements because it's too likely that the student will then focus on the timer more than the goals of practice. However, timers can be used in other ways.
Set your timer for three minutes to work on one difficult section in your music. Set your timer for 15 minutes to play scales and arpeggios. Set your timer for 15 minutes to polish a performance piece. Set your timer for 15 minutes to play only Bach. Set your timer for three minutes to improve your tone on open strings. Etc.
I did this yesterday with my new iPod Touch timer and found it useful because I have the tendency to obsess and only work on one piece of music when I practice. When I limited my time to 15 minutes for one piece I knew I had to be efficient with my practice time and was much more careful - I probably got as much done as I usually do in 30 minutes!