Back in the 1920s and 30s, scientists of the era had suggested that the geological record had a 30-million-year cycle, while in the 1980s and 90s researchers used the best-dated geological events at the time to give them a range of the length between 'pulses' of 26.2 to 30.6 million years.
That 2018 paper, by two researchers at the University of Sydney, looked at Earth's carbon cycle and plate tectonics, and also came to the conclusion that the cycle is approximately 26 million years long. .