Adding a time limit to take a quiz?

There is no way in Gravity Forms to enforce a restriction like that. You could add some JavaScript to the page to make it impossible to change any more form inputs after the time is up, and then offer a popup or some other message to explain to the user what happened. You can then have them submit their answers, or possible submit the form automatically. This would all be done with jQuery.

Here is one way to add a timer to a page with a form:
http://howto.caspio.com/customization/add-a-time-limit-to-submission-forms.html

Here is a discussion of submitting a form with jQuery:
http://stackoverflow.com/questions/1200266/submit-a-form-using-jquery