Think they should spawn about every 5 weeks, and I am sure they do not all spawn the same time so depending on how many females you put in you may have some new fry on a weekly basis.
You have to think that most of them are probably being eaten as soon as they leave their mother, especially with the fish you have. Then again you may find out later this year how many you have when it cools off, assuming it gets cold enough to kill them off where you are.
I have yet to see any tilapia fry or fingerlings in my ponds, but since they are so turbid it would be hard to notice them. I figure if I happen to ever catch a tilapia that is smaller than the ones I stocked I will know they spawned and some survived.
I have even thought about setting out a trap to see if I can get in tilapia in it.
