6 - 12 months. Anything more than 12 months is playing with the girl and making her father look silly. Should be able to determine whether or not the girl is Ms Right during those months. Should let her and the father know what your designs are for her. And if it doesn't "work out" then have the fortitude to let her and the father know that it was you, and not her, for the reason of the courtship not working out in the end.