they will give birth around one month after conception, but as someone said they will still be pregnant and will give birth again in another month to more babies without having mated again. She will get a dark gravid spot before giving birth although my guppies have gravid spots even when not prego although it is brown instead of black. So after about a month after conception and if there is a black gravid spot and if the belly is huge, you should put the female in a breeder net/box unless you do not want to save them all and you have lots of foliage in the tank.