I can give you some kind of reasonable estimate for precisely one kind of work: stuff for which I have good domain knowledge, unfettered access to a small set o

I can give you some kind of reasonable estimate for precisely one kind of work: stuff for which I have good domain knowledge, unfettered access to a small set of stakeholders who are in unambiguous agreement on the need, that is delivered on a technology platform that I know well, using people with whom I've worked in the past, and that has no dependencies on other teams. This has happened approximately three times in my career.