<p>One of the professors I would like to receive a recommendation from was visiting from an institution in another state for the semester I took his class. I asked him through email about two weeks ago, and he hasn't responded. How long should I wait before asking another professor on my campus for a recommendation? What would I say if the second professor said yes, but the first finally responded and I had to say I'd replaced him?</p>
<p>I would send a follow-up email before asking another recommender -- just forward the first email with a polite note asking if the professor has had a chance to consider your request.</p>
<p>At any rate, you can almost always send extra recommendations, so if you have four recommenders instead of three, it will probably be fine. (In fact, it might even be better to have more recommendations, if they're all great.)</p>