The James Bond franchise is an exceptionally long-lived one. Every few years a new Bond film comes out, although there isn't a set schedule for when they do so. However, looking at a chronology of Bond film release years shows a general pattern of 2-3 years between releases if they're using the same actor as Bond. When there's a change in actor, there are larger gaps.
However, the recent films had a 4 year gap between Quantum of Solace and Skyfall. This seems particularly odd given that Quantum of Solace continued a reboot of the franchise started with Casino Royale, so a steady stream of frequent films is a good idea. Indeed, between Casino Royale and Quantum of Solace there's only a 2 year gap. This makes the 4 year gap between Quantum and Skyfall all the stranger.
What's the reason for the delay for making Skyfall?