I agree that it's rude and disrespectful to leave a duty as soon as you join unless it's an emergency, however I don't think Mentor's deserve a longer penalty for doing so.
The terms and conditions for being a Mentor are pretty vague. Some people are also crafting/gathering/PvP mentors. Of course we're expected to know a little about the classes we play and the dungeons we run but we're not obligated each and every time to run people through mechanics, and although it's frowned upon, we can leave if we want to leave.
Before anyone quotes me, I have never left a dungeon unless I've had to, and I'm always helping people whenever I can. I just don't agree with a longer penalty than any other player - we shouldn't be punished more than anyone else purely because we're a Mentor. Especially since emergencies do happen.
I forgot to mention - I've had a lot of people who have either insulted me because I've tried to help, ignored help, or have trolled the duties I have been in. I don't see why I should get a harsher penalty than those people if I wanted to leave because of this.