How long has to be left on my passport to travel to Tunisia?

You can use your passport all the way up to the day it expires. Although many countries require validity for the length of your stay, according to the US Department of State, to enter Tunisia, your passport just has to be valid on the date of entry, and you have to have one blank passport page for the entry stamp.

Of course, I would suggest that you make sure it is valid for the length of your stay so that you don't have problems trying to get back.