Let me think...

My first ever molly fry were born just before last Christmas, December 21, 2004 to be exact.

In February I gave some to my nephew that I thought were all female, but one later turned out to be male...Hmmm, how long was it before we could tell that?...

Somewhere around the 4 month mark. March-ish, from December fry.

And before the gonopodium formed they looked exactly the same, fanlike fins and all.