What is a gender-neutral alternative to the expression "man-days"? I thought of "work-days" but am wondering if there might be another term. The use would be, for instance, "This project requires staffing of 1000 man-days over a six-month period."
Person-hour or person-day are gender-neutral alternatives suggested by Wikipedia.
If we’re talking labour, you could use worker-days, because in this context, worker is a better fix than person.
For software efforts, I like "developer days". Sure, it's longer, but it alliterates nicely, and can be abbreviated to "dev days" once everyone in the organization is familiar with the term.
We use labor days. The man-hour has become the labor-hour.
Surely man hours or man days are both acceptable. You say you want a "gender neutral" alternative, but these forms are unmarked, much like we use mankind when talking about all people, not just those who are male. If you use a term like people days you might risk being ambiguous and waste time explaining your new word that could be spent on this massive project!
I sometimes use the phrase "days of effort" as in:
This task will take about 20 days of effort.
Since the phrase "staff years" is fairly common, I expect it would be well-understood to say "staff days". This avoids the ambiguity of "work days", which could be understood to mean Monday-Friday (depending on how clear the context is).
Effort-Days has been my best and I have used it for over 30 years now. It is also recommended by a number of multilateral and multi-national organizations.
Assuming the context is labuor generally, I would suggest "work days".