Re: How long did Nolans Batman actually fight crime?(SPOILERS)
He comes back Gotham at age 29 and creates Batman, fights Ra's on his 30th birthday, and then he takes the fall for Dent's crimes and disappears (presumably) less than a year later. In TDKR, that was the last known sighting until he returns 8 years later for a short period of time and then fakes his death.
Anyway, Nolan's Batman is a deconstruction of the idea of Batman. It's a disease Bruce Wayne had to be cured of in his point of view in order to move on with his life, and it was better for an ex-cop that wasn't driven by the same things as Bruce to presumably take over his job. And of course, in order for Bruce Wayne to move on with his life, he had to fake his death and exile himself from Gotham entirely--which is what Alfred wanted. Not too happy with this or the logic behind some of it, but there it is.