The maximum duration of menses is ten days. If it exceeds then it shall be called the blood of haiz as per routine and the extra days shall be of istihazah. In the question mentioned above, your routine is of seven days, so the next five days will be considered istihazah. Then if after ten days of purity (total fifteen days of istihazah and purity), the blood which came seven days will be counted as haiz, it is mistake to understand it istihazah. Yes, the blood which comes after seven days shall be of istihazah provided it exceeds ten days. If it stops on tenth day or before then it shall be considered that the routine of haiz has changed.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband