It depends on where you are living.

If you are living in a third world country, or a developing country, you might need a lesser amount of Bitcoin to retire. On the other hand, if you are living on a developed country like in the US where prices are much higher than that of a developing country, you might need even more Bitcoins, way more Bitcoins.

Your cost of living is also a factor. How much is your monthly expenses? Multiply it by 12, and then multiply it by 25 years. The total amount that you will get is the total amount of Bitcoin that you need (of course converted from your fiat currency to Bitcoins). Overall, I don't see myself holding that much Bitcoin when I retire. I guess at that time, I already put my money into safer type of assets like those blue chip stocks out there, and maybe some bonds.