Appointment finished early

What does it mean if your ADOS appointment finishes earlier than the time expected to be reached?

Almost certainly nothing. Estimated timings are just that - estimates. Also, they are normally given as "up to" timings.

Did it finish a few minutes early, or did end significantly early?

It was meant to be for 90 minutes, but it finished 12 minutes early.
I couldn't specify the details to the therapist because I felt overwhelmed and I'm thinking of ringing the clinic to tell them some details pass to the therapist, but I'm not sure what to do.
The results will come after a month

A 90-minute appointment finishing 12 minutes early doesn't sound like an issue to me.

If you have any additional information which you feel would help the clinic / therapist, then by all means let them know. However, they've likely got all the information they need anyway.