It really depends on the length of the appointments, how much information has been shared via paperwork ahead of time, the skill of the diagnostician, whether data from other diagnosticians (such as speech or OT) are needed, and other factors such as whether there are co-morbid diagnoses, etc.
Oftentimes a skilled diagnostician who has expertise in ASD's will have a good idea whether the child's dx lies in that direction or not after an initial appointment (provided it's long enough). We had a 2 1/2 hour initial appointment after I'd already sent in a mountain of paperwork and I left there knowing the doctor felt my son Autistic Spectrumish, and leaning in the direction of NonVerbal Learning Disability. From there had us come back for a one hour appointment at which time he did a diagnostic test and referred us on for speech/language assessment. After that we had an appointment in which he pulled it all together and gave an official dx.
Yup, those were some of the longest weeks of my life too. I think the waiting was the worst.