I trusted my therapists (the ones that I worked with a long time) pretty much immediately, but I don't tend to have trouble trusting people.
Interestingly though, the therapists I chose not to work with, it really wasn't about trust so much as personality. I suppose they were probably ethical enough, etc.; I just had no opinion about their personalities whatsoever - they almost lacked personality to me.
The ones I trusted immediately, I guess it was that their personality clicked with me and I was comfortable with them - thus, the trust.?? Hmm.