When do physics REU programs typically reply back to applicants?
I think that a lot of this depends upon the Program. There are REU Programs in "desirable" locations that get tons of applications that must be sorted and evaluated. There are other places that get fewer. There is some matchmaking involved as well, as the REU site director will want to make sure that everyone admitted to the Program has a research home. All of this takes some time.
I heard back from my first REU yesterday. But last year, I didn't hear back from one until late April. Like the above post says, it varies. A lot.
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