COS from H4 to H1B


my question is..I got h1b approval and i wanted to know when it would be the right time for applying COS. i m currently on h-4 visa ad it is valid till nov 2016.Can i apply for COS before oct 1st 2014 r do i have to get it done only after oct 1st 2014. and how long it would take to get COS approval??