After what frequency are there diminishing returns to doing PRP?

You are correct that there are many PRP protocols, many opinions and few protocol studies. However, several new studies  now exist and are being performed  . A study from Beverly Hills has just been presented that shows that initial monthly treatments are superior to treatments every 3 to 4 months. The frequency of Maintenance treatments depends upon whether you are reaching your goals. If you wish to regrow hair and not just maintain hair, more frequent treatments are suggested. If your hair is regrowing, then the maintenance treatments can be separated buy a longer period. There are NO studies to substantiate that 6 - 7 x  is superior.  There are studies that point out that decreased or absent granulocytes and red blood cells are more important than the final concentration. There are NO studies regarding any adverse effects of Lidocaine. If you want to enhance results consider using finasteride, Minoxidil, nutrifol and low level laser. Find a PRP expert. Ask to see photos.PRPHAIRMD.COM 

Frequency of treatment depends on the amount, type, and method of use of PRP.  This is variable in various practices.  Typically approximately 13ml of PRP is needed for each half of scalp.  The type and kit of PRP used is also very critical.  Generally treatments can be every 9-12 months with an optional booster 3 months after the initial therapy has been shown to be helpful. 

Hello Telexfromyou,
Over the long run there may indeed be some point of diminished returns, but that point is likely related to age more than anything else. There's nothing in PRP your system becomes immune to or doesn't benefit from.
You're correct and on track with 5 to 7 x PRP. However, 3 to 4 times monthly certainly won't hurt, but 6 sessions every 5 - 7 weeks would likely be adequate. I'm not seeing the benefit of the frequency of 3 - 4 sessions per month. Maybe, I could justify 1 treatment every 3 weeks for 18 weeks. May help, but how much more is a question. Only health concern would be possible infection at injection site. The more injections, the more risk, but that is minimal risk.
ECM; If you desire to further enhance, try an extracellular matrix (ECM). In light of money not being a problem, consider using a amniotic or placental matrix. Minimum 100mg of ECM per 7mL of high density PRP.  
Medication; Consider adding Propecia and Minoxidil to your regimen. The side effects are applicable only to a small portion of the population. I encourage you to try them. However, not together at first. Try one first and see. If no problem after a month or so, then add the second. Start with Minoxidil.
Hormones; Most importantly, investigate what your hormones are doing. For example, if your DHT is too high, it will work against and possibly inhibit your hair regeneration and longevity of the treatment. 
I very much encourage you to locate and visit a good Integrative or Functional Medicine physician. They are trained to get your hormones and metabolic system optimized. There are a number of methods they can employ to push down inhibiting hormones such as DHT while pushing up beneficial hormones. 
As a side benefit to your quest for great hair, these type of physicians will have you feeling overall much better as well. They are proactive and pro preventive medicine.
Low Level Photo/Light therapy; you can either locate a physician with a light therapy device or purchase a good light cap and use it a few times per week. Don't go with the cheap ones.
Lidocaine; I wouldn't worry too much about lidocaine. It takes about 7 days in vitro for it to kill cells. It is only in your system a few hours. If anything, don't use any lidocaine products with preservative in it. 
Microneedling: After each injection session, be sure to get microneedling. This is very good for the health of the scalp. A healthy scalp is a healthy follicle environment. 
Maintenance; The life of these procedures is generally unique to the patient. The initial cycle of the treatments generally last between 12 to 18 months if not using the prior mentioned medications, photo therapy and hormone management. I don't like to wait this long because it means were starting at or near zero again. It also means more than half of the procedure will only provide benefit in 6 to 9 months. 

Therefore, consider having a maintenance treatment every 5 to 9 months as appropriate for your condition. Sooner won't hurt. Above all, don't wait until you see the restart of hair caliber thinning or loss. Should you do so, you may need to start a whole new cycle of treatments. Don't forget to keep your hormones in check. Doing so will prolong the life of the procedure and is cheaper and easier than the procedure. 
I wish you all the best in your quest for regaining your hair.

Warmly,
Arbella Saris, MD