Schedules are challenging to estimate even when you can count on continuous, stable efforts. That does not seem to be the case here.

First class? About a girl-year or two I'd guess. Fully usable? About half that.

What’s a “girl-year”?

A less male gender focused way of saying a “man-year”, a common term to imply the amount of effort, rather than an exact duration.

If someone says it’ll take 3 person-years (just to make it gender agnostic), that means if only one person was working, it’d take about 3 years. It implies that theoretically 3 people could do it in one year, 6 people in 6 months.

It’s not a definitive statement, especially if one has read mythical man month and buys into it, but gives a very tough scoping.

Interesting choice of words beyond just gender. 'Girl' is roughly analogous to 'boy', both implying reduced age. Considering how proficient teenagers are these days, I suppose the terminology is not that far-fetched.

This is what threw me for a loop. I speak in person-hours at work even when other people speak of man-hours, but hearing someone speak in girl or boy-hours is a bit jarring due to the implications around age and experience.

I didn’t read that much into it. Now I’m curious if the OP had some secondary motive beyond using a different gender pronoun.

My secondary motive is that I'm a girl :)

I certainly wasn't much more than sleepy and wanting to break the stereotypes on that particular expression when I wrote it.

Presumably the amount of work that a girl can do in one year. Similar to the "man-hour/month/year" unit.