My wife is on birth control (approved by a Rabbi) and when she takes a break after 6 weeks her period isn’t a normal flow. Rather she only stains and it never become a classic period that she would get if not on birth control. When does a stain on colored undergarments become halachically a flow and make her into a nidda? I heard in the name of Rabbi Feinstein that the amount is the size of a half dollar bill. is this correct?
The general rule is that if she sees either:
Two fingers worth of blood in a pad three times in a day
OR Three fingers worth of blood on a pad once in a day then you can assume her period started, otherwise she is tehora.
However, even if she is tehora you should be careful since she is spotting.
She can change her pad once every three or four hours, changing twice throughout the day.
This can be a bit confusing, so please reach out to a Rov if you have any need for clarification.
All the best
Rabbi Shay Tahan