Well I have some fry from MOsbourne that I believe are a few weeks old and I can see a gravid spot on one of them, so I would assume pretty soon. It's not as easy to see in mollies as it is guppies, at least not for me. Once they're a little bit older I usually use the gravid spot if I can see it, but the anal fin is the dead give away.
That is not the gravid spot it is their insides. I have molly fry. They get the gravid spot when they have babies inside them. Only The females get it.
My guppy fry are about 3-4 weeks old. They're definately NOT pregnant, but the females already have the gravid spot. I like to keep my males and females separate until I'm ready to breed them and the first way to be able to tell the difference is that the females develop the gravid spot. It takes a while longer for the males to get their anal fin.
I'm not sure about mollies, but that's how it is with my guppies.
I agree, this is definitly NOT just her insides, there are 2 fry and only 1 has this. As mentioned, it takes a bit longer to be able to tell by the anal fin, but once there a couple months old, that's the dead giveaway.