The gravid spot will get darker the further they are. Typically they loose their appetite very close to giving birth. When I bred guppies, I would keep the pregnant female in a breeding box when the gravid spot was very dark and she stopped eating. I would have babies within 24 hours. You shouldn't keep a pregnant female in a breeding box or breeding net for more than 24 hours. My females would also rest their big belly on the bottom of the tank.

Sounds like you should have fry very soon, although I can't tell you specifically how many days. Keep you eye out for the little ones, and be sure to remove the female after she is done giving birth or she will eat her babies.

Hope this helps.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."