That is the thing with Bitcoin we are waiting for the future but we do not even know if the waiting is going to pay off.
I agree, it all depends on the Bitcoin price when you decided to retire. If you happen to have 400 BTC today, I can say you can retire tomorrow because of it's price now. And visualizinig that after 20 year you have 1000 BTC but BTC price is somewhere less than 1 USD, I don't think you can retire yet . So yeah it is base on the amount of Bitcoin and your holdings wether it is wht you need to retire in 20 years.