What is a gender-neutral alternative to the expression "man-days"?
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.