Craig seems to be the one actor in the role who definitely doesn't need it for his career. The only thing one safely can say is that Craig will leave once he feels it's not fun anymore. That could take a long time. That said I doubt Craig will ever come remotely close to his sell-by date. I'm confident he won't stick a minute longer than he feels welcome and just move on, once that day has come.
How long exactly Craig will be our Bond is everybody's guess. It probably depends how fast Bond 24 can pick up momentum, but I think the plan to get it on screen two years after Bond 23 is mostly wishful thinking - or BS, depending on how one's personal view of the concerned parties happens to be - three years probably being more realistic. This is a long time, an eternity in the business, and many things can happen in between.
A Bond 25 - yet another two to three years after Bond 24 - with Craig in the part seems somewhat unlikely to me.
I have a theory that four films is EON's magic number. If you look at the box office takes for the Connery and Moore films, they both peaked with their fourth outing, and their films saw diminishing returns after that. Brosnan also had his highest earner with his fourth (and many speculated that a fifth wouldn't do as well, with Broz getting on in years and the overall negative reception of DAD), and that's when EON cut him off.
There's also an audience sell-by date. People get restless and want something new. We see the constant trend of rebooting and starting over in Hollywood (Spider-man and X-men both restarting after three films, and probably Batman too), and with four films basically spanning a decade, that's time for a whole new wave of moviegoers just turning 12 or 13 who've never seen a Bond flick before to 'start over' with a new Bond actor.
Anyway, regarding a director, wouldn't anyone like to see Mendes direct two films in a row? Assuming he puts together a fairly well crafted (and received) Bond 23, why not go completely outrageous and shocking, and hire the same guy twice?