When Bitcoin launched, miners would be awarded 50 BTC for each valid block they found.
The first halving took place on November 28th, 2012. At that point, the protocol reduced the block subsidy from 50 BTC to 25 BTC. The second halving occurred on July 9th, 2016 (25 BTC to 12.5 BTC). The last one took take place on May 11th, 2020, bringing the block subsidy down to 6.25 BTC.
You might notice a certain pattern here. Give or take a handful of months, a new halving seems to occur every four years. That’s by design, but the protocol does not set specific dates on which a halving takes place. Instead, it goes by block height – every 210,000 blocks, a halving occurs. So, we can expect it to take about 2,100,000 minutes for the subsidy to halve (remember, a block takes ~10 minutes to mine).
In the above chart, we can see the decrease in the block subsidy over time and its relationship with the total supply. At first, it may seem that the rewards have dropped to zero and that the max supply is already in circulation. But this is not the case. The curves trend incredibly close, but we expect the subsidy to reach zero around the year 2140.