I've recently learned that scents can trigger memory recall in humans, and am thinking about setting up an experiment where I would attempt to induce memory recall through exposure to scents.

In order to help reduce the amount of experimentation required to actually trigger recall, I'm interested in - What kinds or qualities of scents have been demonstrated to trigger memory recall in humans? Are they pleasant smells/unpleasant, sharp or subtle?

Is there any information about how long of an exposure is needed to trigger such recall? Is it 2 seconds, 15 seconds, an hour?