From reading up on the subject a bit I've learned that the non-inheriting children of the nobility would often become either soldiers or members of the clergy, both of which limit their chances to have legitimate children, but obviously a great number of them would choose neither, marry, and have children, to whom the "soldier or clergy" career paths would also prevail - but again, a great number of them would choose neither, marry, and have children. Basically what I want to know is how many generations would it take before your noble blood was essentially irrelevant?

Also - I'm aware that if non-inheriting sons enjoyed any degree of military or civic success, many of them would be created nobles in their own right, but for the purposes of this question I want to exclude them.