What is the appropriate injection depth for PRP for hair loss?

This needs an in-person evaluation. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

In  my opinion, the injection depth should be subdermal. At this depth, fewer injections are needed because the PRP can flow in the subdermal space. This allows more follicles to be treated with the same amount amount of PRP. There  is less chance for pressure overload and it is less painful. This injection depth was proven effective in a study just published by Doctors Hausauer and Jones in Dermalogic Surgery. 

Needle length is not important. Simply being just subdermal and parallel to follicles is optimal.
There is now a new exosome product I'm in trials with that I expect will hit the market in the near future. The benefits of it are: 1) if there is a follicle stem cell in the area, it will be activated and grow a new  hair follicle. PRP cannot, 2) Like PRP it will rejuvenate and regenerate any distressed or unhealthy follicle, but do so much more quickly, 3)  More youthful hair color is obtained, 4) Less than half the number of injections are required, 5) Needle gauge required is a fraction of that required for PRP, 6) requires only 1 to 2 treatments to obtain superior results to PRP, 7) visible initial results within 2 months, unlike PRP that can take a few more months. 
I predict, PRP will soon be replaced with these types of products. For example, I'm now only infrequently using using PRP in joints, but using exosomes specially created for joints. Results are significantly superior to PRP.    
Warmly,
Arbella Sarkis, MD