Re: "The Dark Knight Rises" Review and Discussion Thread (spoilers)
Something like six months, as I recall. That's also six months where presumably there wasn't much going on in Gotham as far as maintaining public works infrastructure and systems. Pipes, powerlines, tunnels, trains, all stuff that needs to be maintained regularly and won't sit well for six months.
One nod to this is that we do see the city's lights flickering out in places, but only in the aerial shot preceding a conversation between Gordon & Blake and then immediately following that conversation.
"In the future... do I make it?"