In the PEP 602 document (https://www.python.org/dev/peps/pep-0602/ https://www.python.org/dev/peps/pep-0602/) they state that the previous schedule was roughly

In the PEP 602 document (https://www.python.org/dev/peps/pep-0602/ https://www.python.org/dev/peps/pep-0602/) they state that the previous schedule was roughly 18 months. One reason they wanted to forgo this time frame is that there is pressure to stuff many changes into each release because missing the deadline means waiting another 18 months.