I would think cadet branches should be a chance to form upon the character becoming landed, based on a range of factors: Your youngest brother, 4th in line to the throne of France after your kids, isn't going to be forming a cadet branch because you gave him a county in Flanders, but your thrice-removed pagan cousin in Sweden whose just had their claim pushed by a third party will almost certainly do so. Some things might have zero chance (the legitimate children of the dynasty head should never form their own cadet branch while the head's still around) some things might be guaranteed (...unless they earned that title from adventuring) but it probably shouldn't be some cut and dried "X generations and they're on their own" sort of thing.