I don't know how much of what I'm going to blurt out in the next few lines will be relevant... But here:
If you get brown discharges occasionally prior to your periods, you still pray. Once the discharge turns red, you stop praying. For 10 days maximum. If the number of days that you bleed exceeds the 10 days, it's better for you to make ghusl on the 10th day and start your prayers.
its so frustrating when you think you have finished your normal period only to find out the next day it wasn't quite finished..
Don't you have a rough idea on the number of days you normally bleed?