breaks
You just never know. Four to eight weeks is average, but there are so many kinds and degrees of breaks that there really is no average. The last two or three weeks a cast is left on is mostly precautionary for most breaks.
Bone begins to heal immediately, and it actually heals very fast. It also usually heals stronger than it was before the break. All told, I've had almost twenty broken bones, and casts have been left on for as little as two weeks, to a high of three months. The three months was for an arm broken in several places, and a broken elbow.
I once had buckle fractures in both wrists at the same time, however, and only had to wear casts for two weeks, but was told to do nothing strenuous for two weeks as a precaution.
So much depends on the severity and location of the break that time is tough. I've had a number of broken bones that couldn't really take a cast, and they generally healed well enough within a couple of weeks that I was 100% pain free and functional again.
This aside, most people with a broken arm will be in a cast for six weeks. If, however, they remove the cast after four weeks, or even after three in a young, healthy adult, the bone will likely be very close to 100% healed. I've seen X-rays of breaks that were only seventy-two hours old, and the degree of healing was amazing.