i think it's roughly 4 weeks,
you'll notice that she will get a dark spot near the vent area,
it will get darker over the time,and she will get bigger,
she may take to hiding in plants to get away from other fish,
when she is near the very end you can put her in a breeding net,
or leave her to give birth in the main tank,if you provide floating plants
and plenty of cover there will be a biger survival rate,depends if you
want the fry to survive or not,after they are born they feed from their egg sack(not sure how long thta's for)then you can feed them crushed flake or fry food.