Answer

If your habit is six days and then in that particular month you bleed more than ten days, your habit will remain the same i.e. six days and the surplus blood will be considered as istihadaah.

However, if you bleed more than your habit, but less than ten days, for example up to nine days then the following month your habit will be nine days. (Fatawa Hindiyyah p.37 v.1)

Only Allah Knows Best