21 year old with AGA. How long do you think I will be able to maintain my hair with rogaine and PRP?

I recommend seeing a board-certified doctor with experience using PRP for hair growth. Prp is effective, but I have a very specific protocol, every 3-4 weeks for 6 months with prp therapy. I also add in progesterone to speed the healing process.  After that the injections are tailored down. You should also be on viviscal and other growth promoting ingredients. See an expert for a formal evaluation. Best, Dr. Emer

As  women's hair loss tends to be multifactorial, I agree with Dr. Donovan, it is difficult to predict how any individual treatment will work. You should be monitored by a hair loss expert for all of the variables  mentioned. We have found that PRP is very effective in younger individuals. If topical Rogaine and PRP are not able to control your hair loss, you can switch to  oral Rogaine (compounded). Topical anti-androgens are not as effective as the oral. In studies ( Sinclair) low dose spironolactone with oral minoxidil is  very effective. Work with your hair loss expert on a treatment plan 

How long do you able to maintain your hair with rogaine and PRP is difficult to answer because it is a multifactorial genetic process.  İt is better to  evaluate your family members who has AGA that what is the norwood grade they have, when they started to loose hair etc.