For Canada and most states east of the Mississippi, it is max 41' kingpin to center of tandems. I have been with some large carriers and have seen very little enforcement of this (but I do see a lot of trucks running way beyond the 41 mark).
Personally, if I had to choose between overweight and length, I would take my chances and run a little bit past the 41 if needed to keep axle weight legal. Having said that, I do recall one of our trucks being charged 6" past (41-6) at Sarnia.
California is 40', kingpin to center of rear axle and of course they do take enforcement action.