Student or Learner
is this sentence correct?
Please advise as to when I can receive a response to my letter.
Correct grammatically, but not the best choice of words, IMO. The person you are writing to will not have complete control on your ability to receive the response (the word can), due to possible delay in post, etc. Better (and more polite) to say something like: Please advise as to when I might (or should)expect to receive a response to my letter.
It's tempting to suggest Please advise when I can receive a response to my letter, and that would probably be clear enough, but it looks as if you are asking for the advisement at the time you can receive the response (please advise when- please advise (something, we don't know what) at the time...).