Asalamuilikum,
My wife is having abnormal periods for last 3 months. The period start with some spotting and after 4 to 5 days bleeding increases and lasts up to 15 days and even more up to 18 days. At this time she is not taking medicine and doctors said because she is feeding for 8 months baby its normal. Until how many days she will count as periods and after how may days she can pray and after how many days we can have sex.
الجواب وبالله التوفيق
As per the situation described in your question when the period starts during the usual days (of her habit) then she should consider the first ten days from the start as haidh and refrain from salah etc. during these days. After the completion of the ten days any blood that continues will be considered as istihadah(i.e. spotting, dysfunctional uterine/vaginal bleeding). Until a fix habit does not develop the above method shall be used.
واللہ اعلم بالصواب