it will take the same time as a new petition. In regular processing it may take 2-6 months and in premium you can get the decision in 15 days. If it is filed before the expiration date then you can continue to work and stay here till you get a response from USCIS.
once the extension is approved and if you leave the country you will have to appear for the interview. If the client, employer, location everything is same then you may also be eligible for Interview waiver program in which you will just get the new visa stamp without having to give another interview while in India but you would still may need to give your photograph and fingerprints.