If you are a licensed clinician weighing up whether to add ADHD or autism assessment to your practice, it helps to separate two very different things: completing an initial course, and being ready to work independently.
Two different milestones
Introductory workshops for most recognised assessment tools are commonly delivered over a small number of days and cover the diagnostic framework, the tools themselves, and initial scoring practice. Reaching independent clinical competency, the point where you can reliably conduct and score assessments without supervision, is a separate and longer milestone, typically requiring several months of supervised practice on top of the initial course. For some tools, a further, more demanding reliability standard exists for clinicians who want to use the tool in formal research settings, which can take longer still.
What this means if you're planning your own training
Budget your time in two stages, not one: the course itself, and the supervised practice period afterward. A provider or platform that implies a two-day course alone makes you a fully independent assessor is understating what the specialism actually requires.