Must one inform the suitor about a long menstrual period, and how should one tell him about it?

If you menstruate for 15 days and then experience purity, you are not obligated to inform the suitor about it. However, if menstruation exceeds 15 days, then you are experiencing istiḥāḍah (prolonged non-menstrual vaginal bleeding). Regarding whether istiḥāḍah is considered a defect that necessitates disclosure, there is a difference of opinion among scholars. The majority of scholars hold that it is not a defect that grants the other party the option to annul the marriage, while Shaykh al-Islām Ibn Taymiyyah holds that it is a defect. If you wish to inform him, you can inform one of his maḥārim (unmarriageable kin) so they can inform him.