If it is continuous and exceeds 15 days, you should go back to your old and initial period.
This means that if you were having 7 days of monthly period in the beginning of the month, you should consider the bleeding that comes to you in the beginning of every month for seven days as your menses and then take a bath and pray.
But if you didn’t have a specific time or you have forgotten it, in this case you should try to distinguish from this bleeding what seems to have the characteristics of menses and stop during that period. If this is also not possible, you should compare yourself to your sisters and females of your close family and be like them in the duration and in the start time.