Listen to the section here about carb backloading: http://robbwolf.com/2012/08/28/carb-backloading-episode-147/
Basically, the deal is that you become more insulin resistant as the day goes on and if you time your workouts and carb refeeds at the end of the day you can take advantage of noninsulin mediated glucose transport to preferentially get fuel into your muscles over your fat cells. I've been doing this for years without knowing what it was, it's just that my schedule worked out that way, and it works really well.
Have a read at these 5 articles, I think in they answer your questions.
I think of insulin sensitivity in a broader context than hours. When I was insulin resistant, my blood sugar was 200 and would not go down with fasting on two repeated blood tests. It was a chronic condition that took years to develop. If you want to do a quick and easy test, measure your blood sugar an hour after a meal with carbs, and again just before your next meal. If your blood sugar drops back to normal (80-100), you probably have good insulin sensitivity.
The Tenacious Insulin Bump 5 Answers