I got this response from a different website. It's a pretty good / well written explanation on the 8-year gap:
"Unfortunately the 8 year issue/Batman stopping has strong roots from the first two films. This series clearly established in Batman Begins that this particular version of Batman's goal is to end organized crime in Gotham and give justice a chance to prevail within the system. This theme is carried through in TDK when Bruce is trying to pass the mantle onto Harvey Dent and retire. Now, it's fine if you don't approve of a Batman who would retire, but it's a bit unfair to bash this film and then call The Dark Knight brilliant when its whole story is rooted in the idea that this version of Bruce Wayne is not only willing, but eager to hang up the cape and cowl and let public officials take over once the system has been cleaned up (the mob taken out).
Also, Miranda/Talia infers in the film that Bruce only became a recluse 3 years prior to the events of the film after finding out that the reactor can be turned into a nuke. So it's not like he was moping around over Rachel for 8 years. Likely, he was still keeping an eye on things in Gotham (Alfred says "You haven't been down here in a long time" which implies he's been active in the cave at some point after TDK, since there was no Batcave in TDK) for a while. Once the Dent Act is passed (when this happens in fuzzy, but a piece of legislation doesn't get passed overnight) that's when he decides to try and help Gotham as Bruce Wayne and takes the energy project. But when THAT hits a dead end, that's when the whole Howard Hughes routine begins. That's how I took it and there's a lot of evidence in the film to support it, I think. Also, I figure the 8 year gap in the story served a few purposes besides just aging the John Blake character. For one, it sets up an older Bruce Wayne who's not at the top of his game, which is part of the premise of the movie: Batman coming up against a superior physical threat...which is really him coming up against his own mortality. In addition, it gives some distance to the events of TDK which was needed considering The Joker couldn't be revisited and Batman's choice to take the fall for Harvey needed time to play out as a worthwhile sacrifice, otherwise you'd piss on the ending of that film. It's also an absolute nod to The Dark Knight Returns, and the Batman coming out of retirement angle. So to me, the Blake angle is just one of the many reasons they advanced the story 8 years. That's how I justify it in my head anyway, Kev could very well be right about why the Nolans specifically chose 8 years. But some passage of time seemed appropriate for the overall story they were telling."