We estimate it based on when we start being evenly-matched against teams with a given average MMR. (As you might expect, our usual pattern is to lose to a test team consistently, then start being evenly matched, then consistently beat them.) The version of this diagram on our Five page also include bars indicating when we started and stopped testing against various teams: https://openai.com/five/.

We don't claim this is perfect, which is why we label the chart "estimated MMR".