In my experience they drop whenever they like. i spent ages comparing sizes, shapes of belly, gravid spots, behavior, feeding habits etc. and while i was watching my super fat guppy (with squared belly, huge gravid spot and didn't have any dinner coz was hiding) one of the thinner ones that i thought was only a couple of weeks gone (round belly, small gravid spot) just popped one out in front of me! while she was chasing a brine shrimp for her dinner of all things!
I've had some great advice from this forum about raising and breeding them so let me know if you would like any info passed on xx