29 April 2015
As per Section 139(8), if the casual vacancy has been arised due to resignation, then the company is required to appoint new auditor within 3 months from the date of recommendation made by Board.
It is not mentioned that within how many days the board should make recommendations. But I believe it should be 30 days from the date of resignation.
Hence, ideal thing is, you can have board meeting within 30 days to recommend the appointment and after that you can have EGM to finally appoint new auditor.