With current inflation rates you will never have enough money for retirements. I guess that you need at least 20 btc to play if safe
Well, there is no ceiling in that sense, the more BTC the amount is, the better in this idea of taking it as a withdrawal liability, the issue is, how much do you need to have to have a worthy withdrawal, it is not a question of 1000BTC or 1BTC to a worthy withdrawal , so in reality it is something (very) individual. Then, in any case a retirement fund based on several assets is always better, so within that 100% of the money that you stipulate, bitcoin should never be 100%.