I think you need to save more than you're age after 20 yrs. let's say you are 35yrs. old now plus 20 yrs. and you are 55yrs. now in that age you are retired, so if you think can you spend 1bitcoin per month, x12months x the years you can leave, after you retire, let's say +40yrs. you are now 95yrs. old enjoying you're bitcoin, I think in the furture the price of the bitcoin is more than $1000 per bitcoin.