My understanding was he went into seclusion following dent's death and that during that time, he messed up his investments by not being there mentally or physically to handle the issues. It was stated that they believed he was injured in some accident and that's partially why he hid, so to me people stopped seeing Bruce Wayne around the same time as Batman.

To that I add, batman and Bruce Wayne both reemerge at the same time. I don't get how no one, except Blake and Bane, would make a connection.