Thanks for your reply.Depends on how quick the VO gets it.
What was the issue? Why did VO ask for a letter?
It was actually pgwp application and I was working on my thesis in some terms and was not taking any courses which is usual in thesis based programs. But officer was in doubt that I was studying part time during those terms. So I just send him a letter of explanation with confirmations from my university and supervisor.
PS all my class mates got approvals right away without showing any proofs. I was a little unfortunate lol.