Yes and no.
You can change the time-limit settings, and Moodle - running on the server - will start using the updated settings immediately.
However, the count-down timer in the user's web browser does not update until they move to another page of the quiz attemtpt (which is not very likely if all the questions are on one page, unless they go to the summary page, then back again). MDL-27570 is the feature request for doing something about that.