The refund process at UA is a little slow. I have had delays in refunds in the past. Be glad you will get a refund at all, even though it may not happen at the speed you desire. I am not defending the UA timing on this, but it is what it is.

Do you have copies of both when your reservation was made and when cancelled to show it was cancelled within 24 hours, just in case you need to offer documentation? (hopefully you will not) Be patient.