How long was Mary Boleyn Henry’s mistress? Did the queen discovered it? Was Mary expulsed from Katherine’s household after that?

We just don't know how long Mary's relationship with Henry VIII lasted, in fact, we don't know anything about it at all. The only reason we know that they did have a sexual relationship is because Henry applied for a dispensation so that he could marry Anne. We also don't know whether Mary did serve Katherine of Aragon, so her life is a bit of a mystery!