It’s not set in stone but both age and difficulty play a major factor.
Generally, when a machine is around 15 weeks old it becomes a retirement candidate. It would be unlikely to retire an Easy machine while releasing a Hard machine. That simply would just mess up the balance of difficulty in the labs. If there were 75% hard, 20% medium and 5% easy like some of the veterans want it just wouldn’t be friendly to the newcomers. All that being said there have been instances that happened sooner, for example, if a new machine is very similar or the machine had poor ratings.
I’m not sure when Calamity is set to retire, but I have started thinking about how the heck I’m going to be doing the video as it scares me and I can feel it being retired soon.