First class? About a girl-year or two I'd guess. Fully usable? About half that.
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.
I certainly wasn't much more than sleepy and wanting to break the stereotypes on that particular expression when I wrote it.