What is the ruling on prayer for a woman whose menstrual bleeding has not ceased for six years, knowing that her usual period was seven or eight days? Should she pray after fifteen days, or is only the duration of her usual period considered? And is there a sin incurred by one who vowed to drink only two cups of coffee but then drank more than that?

If it is confirmed that you are experiencing istihadah (definition) (irregular vaginal bleeding outside of menstruation) due to the blood exceeding 15 days, then you must refer to your previous habit (menstrual cycle) if you know it. It is not permissible for you to wait 15 days then perform ghusl (definition) (ritual bath). If you have missed any prayers during the previous period, you must make them up, according to the majority of scholars.

Regarding the second question, a vow to do something permissible (nadhr (definition) al-mubah (definition)) does not necessitate fulfillment according to the majority of scholars, and this is the fatwa (definition) (religious ruling) acted upon. There are no consequences for not fulfilling it. The Hanbali school of thought holds that whoever vows to do something permissible has the choice between fulfilling it or offering an expiation for an oath (kaffarat yamin (definition)).