Do you know what she means by "assessment"? Does she mean she's deciding whether she can work with you or not, or that she's trying to get a whole picture of you before diving in deeper?

If the latter, I'm not particularly bothered by the length - I am quite sure No. 1, who I've been seeing longer (about a dozen sessions over 5 1/2 months), might still say she was in an assessment phase before deciding how to help me. If the former, that still seems long, and I'd ask right at the start of the next session, either when she'll decide or what she means by "assessment."

It does seem to me that most mental health professionals only take a few sessions to decide whether or not they can work with a client. Yours has either already decided in the affirmative or is prolonging things too much.