According to a post by Digital Spy who was talking to Martin Freeman at a press conference at SDCC that Sherlock will begin filming Series 3 in January 2013. Here is Martin Freeman's quote.
"[We film] in January until about April, I think." "It's a pleasure for us to do. We love it!"So if that is true then we won't be getting the next series of Sherlock until late fall 2013 or early winter 2014. My guess is it will be sometime in 2014 when the new series will air like in January.