Never. This is impossible
Although it is really hard to believe that Bitcoin can cost a million, it might happen. However, many years will pass, and for this only a few BTC should be remained.

The only time Bitcoin will be worth $1 million dollars is when the USD is worthless. I mean, i'd love a million per Bitcoin, but that would put the market cap at a staggering $17 - $21 trillion. Golds market cap is $8 trillion. Bitcoin hitting those numbers are very unrealistic. $100k per bitcoin is within the realm of reality though.
The real question is if bitcoin reached that price what could you buy with it? Inflation can grow very rapidly especially when governments lose control of their economies so if bitcoin at 1 million dollars only buys a gallon of milk then bitcoin has crashed as well because right now you can buy a lot more with your bitcoin, so people need to focus a lot more in the value and what you can buy with bitcoin than to focus in the price.