Even single man-month long "refactors" are something I'm wary of. Smaller changes are easier to test, easier to review, easier to merge, easier to verify are actually improving the state of things and heading in the right direction, easier to pause when your priorities unexpectedly shift midway through cleanup without leaving a terrible mess...

I'm okay with the occasional week-long rewrite of a subsystem, but usually only after I've spent some time coming to grips with exactly why the old one is terrible and have a firm grip of exactly how the new one will be better.