With my last therapist, I never felt comfortable (saw her for about 8 months). With my current therapist, I felt comfortable as soon as she opened the door. For me, she is very easy to talk to and has that balance of genuine warmth and professionalism.

I think it varies for everyone, but I'd say if it doesn't feel right, you know within 1-3 sessions. It might take longer to feel comfortable, but there's a kind of knowing that you can work with this person.