Exercise and Dopamine Receptor Upregulation

I struggle with chronic fatigue (not cfs) and ADHD.

Stimulants so far have never worked for me. I have low cortisol and am being examined for possible adrenal insufficiency (insulin tolerance test)

I've aksed a very knowledgable person on here (who's also a vendor) and he said I might be suffering from dopamine receptor downregulation, so I've never gotten the DA effects of stims, only the NE related side effects

How long does it take for consistent exercise to induce dopamine receptor upregulation and increase my dopamine levels so I'll react to stimulants?