For ADHD, very quick and easy (however I have been told that is not normal). I met with my PCP and said I wanted an ADHD diagnosis to get accomodations and emphasized I didn’t want a prescription, cause I take enough meds as it is and would prefer to learn how to cope without the use of meds. I also explained to him my history grappling with my ADHD especially in school. The important thing here was showing it is a long time and consistent problem, one that exists in multiple settings (not just school, but at home, and work too). All of this made my doctor feel comfortable giving me the diagnosis. For autism, I haven’t started the process yet, waiting on a referral cause my doctor wasn’t comfortable diagnosing it.

If you want meds to help manage ADHD it will probably be a little harder to get the diagnosis, but I think as long as you establish that it is a long patterned history in more than just one setting, establish clearly why you want the diagnosis, and give detailed examples and stories of the symptoms (and maybe a little bit of luck) I think you should be fine.

I hope this eases your worries a bit and I hope it helped. Rooting for you.