If this is usual and it comes for 15 days every month, this is menstruation and a woman shouldn’t pray or fast. But if this exceeds this and it never stops or rarely, this is not menstruation and it is Istihada which doesn’t prevent a woman from praying or fasting. Here is a similar question and answer that may clear what such a woman would do.
It was narrated that ‘Aa’ishah said: Faatimah bint Abi Hubaysh came to the Prophet (peace and blessings of Allaah be upon him) and said: “O Messenger of Allaah, I am a woman who experiences istihaadah and I do not become pure. Should I give up praying?” The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “No. That is just a vein, and it is not menses. When your menses comes, then stop praying, and when it ends, then wash the blood from yourself and pray.” Narrated by al-Bukhaari, 226; Muslim, 333.
Narrated ‘Aisha: Fatima bint Abi Hubaish asked the Prophet, “I got persistent bleeding (in between the periods) and do not become clean. Shall I give up prayers?” He replied, “No, this is from a blood vessel. Give up the prayers only for the days on which you usually get the menses and then take a bath and offer your prayers.” (Sahih Bukhari Book #6, Hadith #322)