Typically I need 4-6 weekly sessions to decide if I want to commit to something longer. Sometimes it's easy and the first or second session is enough to know it won't be a fit. Usually though, we need to get past intake and into some typical but light issues to see how they respond to me.

Identifying a definite lack-of-fit is much easier than a definite fit. I don't think I've felt sure that anyone I worked with before my current T was definitely a good fit, and it took probably 12-18 months of weekly appointments to decide that with her, so... I'm slow to trust. YMMV.