I'm in Ohio, in the US.
My "assessment" was actually very simple: I talked to a doctor who knew what autism was because she had an autistic son. She diagnosed me after a few appointments.
However, the full evaluation took longer. I went to a neuropsychology testing center and did about eight hours of testing, including testing of things like memory, concentration, manual dexterity, hand-grip strength, and academic achievement, as well as a full IQ test and personality questionnaires. The tests were fairly straightforward. Some of them I was good at; others not; but it was relatively easy to know what I was supposed to be doing. I also had an hour-long interview with a psychologist. I didn't think he did too bad of a job, mostly because he didn't need to do anything other than confirm what we already knew. He wasn't an ASD specialist. I had a minor meltdown toward the end of the day, but I did not hurt anyone or hurt myself, and was able to get to a restroom and calm myself down without trouble.
I also had a two-day career assessment, which included some more academic testing, some interest/talent questionnaires, and some testing meant to test some of the skills you might use at work, such as typing or looking for flaws in machine parts. Once again, the instructions were relatively straightforward, but I found I was very good at some things and very poor at others. The tests involving concentration and multi-tasking were especially frustrating.
For both tests, I insisted on receiving a full report on the results, and these have been useful both for self-knowledge and for obtaining accommodations from the school's disability services office.
I don't know whether your testing will be as extensive. It could be anything from an hour's interview to multi-day testing, or anything in between. But I guess the best thing I could advise you is to be honest about yourself and your daily life (we are often so used to faking NT that we forget that it's possible to tell people we do struggle with things), and not to worry too much about the testing--the instructions tend to be doable, and they haven't got anything against your asking them to explain again if you don't get them the first time.