One of the major factor would be to consider a sufficient reward for miner in relation to price, period of time the reward will last and deflation. If you consider the amount of reward that'll be sufficient for miners in relation to price of Bitcoin and make the reward last as long as possible by estimating how long the Bitcoin system could possibly last in relation to human civilization and changing tech ... maybe you say in the next 200 years.  As an early Bitcoin developer, Just multiple the time by the number of BitCoins that will be issued every day and you get your answer.