Assalamu alaykum

My case is that i get my menstrual period quite frequently, usually twice in one month. The number of days is usually 7 days, but the time is not fixed, so i will bleed for 7 days but my period of purity differs. So sometimes i am clean for 12 days, sometimes for 18 days and sometimes for 3 weeks before my next bleeding. Now, some scholars state that the minimum period of purity (clean period) is 15 days, and some say 13 days, so if you start bleeding before this time it is not considered menses, it is istihadah. Scholars have differed, some say the period of purity is 15, some say 13 and some say there is no minimum, rather if the blood has the characteristics of menstrual blood then it is menses, regardless of how much time has passed between it and the previous period.

My personal view is that i am more at peace with the last opinion, because sometimes it happens to me that i have my period, and after 12 or 13 days i start bleeding again, and it has all the characteristics of menstrual blood (dark, thick) and i bleed for the usual 7 days, so i do not think it is istihadah. So i wanted to ask what you think is the most correct opinion, is there a minimum period of purity between menses, or should a woman act based on the characteristics of the blood she sees.

JazakAllahu khair