Between 2030 to 2040. That's a good 10 to 20 years from now. By 2032, more than 99% of bitcoins will have been mined. That's when demand will outpace supply, as it will take a hundred years after that to mine the last percent.
Sounds a good estimate based on supply, I was about to base my answer on demand or rather the value of dollar. The idea of 1 million also includes great movement in Dollar value as it adjusts to no longer being the global reserve currency. That is a normal phase change thats happened many times previously with the Spanish and British all the way back to the Romans which also reduced the quality of their currency over many decades and centuries, all rising and falling in influence.
So I would look toward Dollar index or DXY which I would expect to be making new lows. Right now thats not true at all, the world has swung round to hold many dollars a bit like how people note Bitcoin dominance. Its kept as reserve assets so that Dollar has a standing in many countries, in many central banks as the asset that backs each national currency. For example Japan a great exporter and advanced nation is holding $1 trillion in dollar treasury debt and that sounds alot but the trade done each year is greater. For me to think 1m BTC is possible then I would have to see these various holdings of dollar no longer exist and much more distribution of value for global currency varied across many different assets types.
BTC will likely rise in better prospect when Bretton Woods is more in question. This agreement extends past my lifetime, it is the ground I stand on in terms of how much influence it has that I've always taken for granted, when the world changes that much then any price is possible.