H1B: Start to end duration..

[blrmaani]

Dear all,
I am currently working in US and would like to renew my Visa. Planning
to get an appointment in Chennai for the stamping.

Question:
The vfs website says that if stamping is successful, the passports will
be mailed to your address. If I appear for the interview on say March 1st (and
assuming that its successful ) when can I expect my passport at my
residence?

If I am visiting India for around 20 days, what will be the good buffer ( in days ) to keep after appointment ?

thanks in advance
regards
Blr