I think it will largely depend on services in your area and the practice guidelines. I think I was the first patient they had referred for ADHD. Also Specialists able to diagnose ADHD in adults are few and far between I think there is one in the whole of our region so had I managed a referral it would have been an extremely long wait.
Adult ADHD still has a long way to go in being recognised and accepted by the medical profession. I have had two medical professionals tell me ADHD is just for children.
The main thing a diagnosis has given me is the ability to forgive myself a little. I have certainly lived fast and loose in the past and have sabotaged so many opportunities. I'm bright, was predicted a great future academically and failed or dropped out at every turn. My mantra was "I am a bad person" as I just couldn't seem to do what to others came easily. Now I see what challenges I've faced instead with my dodgy cognitive functioning!
Taking the drugs initially (stimulants which I had to give up because of side effects ) was an absolute revelation. Suddenly the thousands of thoughts in my head quietened down - all was still, I could think and concentrate. I realised this is what it's like in the head of a normal person - All the time I had been battling against a tide that wasn't there for others.
That said I actually missed the crazy :)
The drugs I am on now are not as effective and I still have side effects but they quieten the tide a little to allow me to write a letter for example without jumping to do several other things at once. If anything they are worse for motivation though hence why I'm still on here!