For every 2BTC per month you want to have for a 20-year period, at the end of which the portfolio is zero, you need a lump sum of 40BTC. Thus, if you want 20BTC per month, you must have a lump sum of 400BTC. If you feel like you have really good genes and expect to live 30 years in retirement, then the present value of that stream of money must be 55BTCper 2BTC or 550BTC for 20BTC per month.  If you keep everything the same, but assume a 6% positive return, the lump sum required is 36BTC per 2BTC for a 20-year payout and 470BTC for a 30-year span.