I would estimate 10-20 BTC
Seems very low to me. Not sure from which country you are from but I would assume you need at least 1-2btc a month at current prices to cover all living expenses, pay taxes, enjoy leisure, etc. You would need these 10-20 per year just to live.
Of course the bitcoin price will rise and you will need less coins,but the living expenses are rising alsim
It depends on the bitcoin price. I reckon the bitcoin price will double every two years, so that price will be $500,000 each then.
I don't think that is possible, we don't know whether bitcoin will exist that long or not and even if it survives its very difficult to touch that price.
To double every year would be similar growth to bacteria, exponential and unrelenting as if it had no competition or flaws. Doubling every other year would fit better as this is the general growth of technology and has been for decades.
Really when its price we dont know as it relies on demand and extension of the network to allow spending also. The more people that use bitcoin, the larger amount holding it between them and not just converting back to dollars. Simply holding and doing nothing is not a substitute as this wont help the demand.
We have world population growth, its not doubling but in theory bitcoin could double as so many do not use bitcoin now. It cant do that for 20 years though or it will exceed the world population by then. The maths is 2^20 or current bitcoin user base times 1 million. That'd take a user base now of 6,000 to 6 billion in 20 years and we already got more then 6,000
So if demand isnt rising that fast, I doubt price will
or increase morethan 2000$ so you just need some amount of btc, risk is always there no one really knows what is the price at that time. 
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