Not a "percentage", but eye contact should be maintained during the introduction (when the teacher is concentrated on you) and during the conclusion - both aren't hard to memorise, and leave a good impression. Throughout the body of your speech, you should look at your audience every sentence or two.
One of my pet peeves though is when people read a sentence then look up at the end. Do not do this. One method I like to do is to memorise the first sentence on each palmcard, and specific textual examples like quotes, so that more meaningful eye contact can be achieved. That should suffice for full marks in that area.