Once on a seminar visit I had dinner with a former student of the most productive researcher in my field. And the advisor has been the most the most productive researcher for an incredibly long stretch of time—decades. The former student explained that the advisor’s rule was to write one page a day. No more, no less.
I have never achieved that. Nor will I ever. But just one page a day doesn’t seem to hard in theory. In practice though. Ouch.