Separating Dempsey Fry from Parents?

After about the 4th time My Dempseys have managed to keep some fry alive.
There has been a dramatic die off rate and of over 300 free swimming
larvae about 50 remain up to a size of about 7-8mm. This was dye largely
to the fact that I was prepared with my egg crate tang divider after their
last attempt.

My question is this:-

How long can I leave the fry in the tank with the parents. At what stage
do they lose interest? I want to be prepared to remove them before this
time arrives, but want to leave then in the tank for as long as possible.
These fry have grown at a much faster rate than those from the pairs very
first spawning that I removed as soon as they got free swimming.

Thanks

Trev