Docg, if you're wanting some to eat, starting now will be fine. They stop, or seriously slow down spawning in ~70 degree water and begin getting lethargic at around 60-65.

If you're thinking you need to start removing them because you're worried about dead fish floating up, it's doubtfull any will. Your larger fish will eat nearly every tilapia in the pond, even tha larger ones will get torn into smaller bites by catfish or turtles.

Waiting for the water to cool and fishing for them in the sun-warmed shallows works pretty well.