I'd be shocked if he made it to six.
He'd be pushing things at that point.
I imagine Spectre will be an enormous success (perhaps even doing better than Skyfall), so Craig will almost certainly be courted back for Bond 25, which would likely hit screens in 2018.That would put him at 50. If he did Bond 26 in 2021, he'd be 53 when he departs the role. That's not quite up to "old man Rog" standards, but it's still pretty old (that's Connery in Never Say Never Again territory).
That said, with the character-centric focus of Craig's era, along with the fact that Craig's films have already acknowledged Bond's age, maybe they'd do something interesting with a Bond who hit the automatic retirement age for Double-Os.
Which he already has hit now. So much for realism.....everything more than Bond 25 and we are back to geriatric Bond
In all fairness it must be said that Fleming, in setting the mandatory retirement age at 45, was
a ) writing in a different age when men and women in general tended to lose their physical prowess sooner and probably also to a greater degree and
b ) took a mode de vivre of severe indulgence for granted, a habit that was bound to damage any person's health soon enough.
Today, with the right kind of balance between regular sports and rest and recreation, with a reasonably healthy lifestyle that avoids self-abuse and neglect, it is possible to maintain a high level of physical capability and efficiency. I am pushing fifty myself, was a former marathon runner and used to do between 30 and 50 miles (in marathon preparation) each week for decades. Age does slow down as well as wear you out, that much is true for anybody and you can't cheat your way around that fact. But a well-trained and maintained agent of, say 55 would be able to beat many younger competitors if they are not trained just as well and dedicate the same amount of time and passion to honing their skills and abilities.
That said, I'd argue the 45 retirement age for 00s isn't meant as an unbreakable barrier to keep chubby agents out of the 00-section. The way Fleming describes it it seems to me to be more a kind of finish line, a goal to set your sights on when you run the risk of burning out. A natural limit that, once it's reached, it's understood you did your duty, carried your share of the dangers and can now relax at a comfortable desk job without having to fear M's voice on the red phone. I could well see Bond working 00-duty beyond that age, provided his motivation remains strong enough.