In this thread, Peter linked an interview he did in 2016, which includes this exchange he and the interviewer had:
I’ve really liked your recent major updates, and especially when you released the new icon for your application. I use the icon a lot on my blog site. So in terms of updates, what’s your process for working out the upgrades? Do you work on coding a little each day, or do you work on it in spurts? Do you beta test features with a few users? In general, what’s you overall process for doing development work, and how long does it typically take to get new upgrades developed and released?
The process for each day tends to be similar. Usually I start by clearing the support (and other) email, including the forum, and then the remainder of the day is spent programming.
The overall development cycle for Keyboard Maestro follows a fairly consistent pattern over a period of about two years (preferably more like 18 months, but it has tended to be more like two years). After a major release there is a lot of general support and maintenance work, so not much programming gets done. Fixing up any issues that were missed in testing and polishing any rough edges. This continues on through several minor and bug fix updates over the next year or so, rounding out some or the major new features, or adding other minor changes. Each of these releases is beta tested by a small group of customers.
After that work starts on the next major release. By that point, I usually have the outline of the major features I want to do. I usually try to have a good mix of engine features (new facilities that Keyboard Maestro can perform) and user interface features (improvements to the editor and the look of the various features). Because a lot of Keyboard Maestro is in flux at this point, I usually just work on it alone with only me beta testing. When the major version is mostly complete, then I start formal beta testing while writing the documentation for the new version. The beta testers inevitably find some issues, but more importantly they point out features that would better complete or compliment other features. When everything is ready, the new major version is released and the cycle repeats.
That's nearly a decade old, so any portion of his process could have changed, but the major-upgrade release schedule has held to roughly two years since 2011 (version 5) according to the information Airy posted.