It probably depends on the dr. I was diagnosed by a dr. who specializes in bipolar and is world-renowned for his work in the field. He not only diagnosed me in an hour after about 15 minutes he was able to predict the patterns that my moods fall into and since I have dysphoric manias and long, long mixed episodes and rapid cycle that's not particularly easy to do. I had to have someone there for him to verify what I was saying and he did that by looking over my shoulder for her to nod or shake her head at times and he really had pretty much everything figured out in minutes although the entire assessment was closer to an hour.

The pdoc I had before that never diagnosed it but when I went in and said "I think I have this because____ and my therapist agrees" she changed my diagnostic code. So I guess she diagnosed it in about 8 minutes which was all the time she ever spent with me. Except I don't think she'd have caught it ever, even if I were still seeing her now (14 years later). Even after changing the code she put me on meds that were totally wrong for someone with the symptoms I had (very manic, probably mixed, highly dysphoric, only mood stabilizer was lithium which wasn't helping (put on that to decrease suicidal ideation) and she added a ton of prozac which caused akathesia AND more severe mania until I finally came down to earth for 5 minutes and realized what was happening and quit taking it and got myself to the specialist.)

My current dr I'm fairly sure would know in a session but would probably see someone several times before making a diagnosis because she is cautious like that. She likes to be 100% confident before she says anything.