How long do you think it takes

Varies depending on where you are. You may have a right to choose clinic nearby with shorter waiting times (Google NHS right to choose ADHD)

theres a chance the waiting list is that long in your area, but i dont think the testing process itself would take that long. i did mine in a matter of a few months, from memory, but australias healthcare system is better than, say, the us (or even the uk honestly)

Mine took 2 sessions of 1-2 hours each for a full adhd assessment

I went to a talk by an NHS ADHD psychiatrist and she said to get on the waiting list whatever the doctor says, then think about paying for private if you have the money. But I did see an article about some private doctors diagnosing people really quickly without the proper deep checks, so it's hard to know. I've been putting getting on the waiting list for years and I could have probably had an appointment already, so keep on pursuing it!

Woah! I know that the waiting time is long because there are so many people on the list. I even flew out of state to get my autism diagnosis.

Where is this?

I was diagnosed as a child, but that took maybe a couple of months to get everything needed for a diagnosis. It was also a diagnosis from a family doc and not a psych so I'm not sure if that makes a difference. 4 years sounds way too long, whether that's the norm for adult diagnoses or not