How much time it takes for Vermont Centre for RFE approval

Hi ,

I got RFE and it was received by vermont centre on 21st August.

How much time normally it takes for RFE responce by Vermont center?

also In case teh RFE gets approved and it takes 3-4 months for approval, from which date the visa is valid , The date RFE got approved or 1st October 2015.