Praise be to Allah.
If the period lasts longer than usual, the correct thing is to follow what is usual. If her period is usually five days, but this time it is six or seven or eight, the right thing to do is to refrain (from praying etc) so long as the blood is flowing, and regard it as menses, and not pray or fast, and it is not permissible for her husband to have intercourse with her in this case, because the period may be longer or shorter; this is normal among women. But if she bleeds for longer, she should not pray and fast, and if it is shorter and she sees that the period has ended, she should do ghusl and she becomes permissible for her husband, and she should pray and fast. This is the correct response in this case.
But if she becomes pure and does ghusl, then she sees yellowish or brownish discharge, this yellowish or brownish discharge is not regarded as anything (i.e., is not menses), so she should pray and fast, as Umm ‘Atiyyah (may Allah be pleased with her) said: We used not to regard the brownish or yellowish discharge that comes after tuhr (becoming pure) as being of any significance.
If she does ghusl after her period, then sees yellowish or brownish discharge, this does not count as anything or prevent her from praying or fasting or having intercourse with her husband. But in the case of pure blood that is clearly blood, then she should stop praying etc., even is it was interrupted. Whether it is continuous or interrupted, she should stop praying etc. according to her regular cycle. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)