Tim Ferris has a routine for writing and has indicated "My quota for writing is two crappy pages a day." and "If I don't write my two pages, I don't panic and go into the spiral."
(summary); possibly worth watching video for verifying quotes and pulling out additional practices.
Note that this piece seems to indicate that his writing practice includes an idea of doing "morning pages", but this implication is likely false as Ferriss likely isn't doing this, but writing toward productive goals rather than to "clear his mental space" as is usually implied by morning pages.