I normally use about 2 oz of Fe-Cl for 9sq inch of copper, I warm up the Fe-Cl using a 60 watt light bulb for 10 minutes, then I hold the PCB at a 45 angle with one hand (rubber glove) while I wipe it using a sponge soaked in the Fe-Cl, I turn the PCB around after about 30 seconds and wipe the other way and I am done within 1 to 2 minutes.
I used to just leave the PCB in a small bowl with half inch of Fe-Cl, rocked the bowl every minute or so and it was taking about 20 minutes.
I have a feeling if you get the results you showed after 80 minutes you have a contaminated Fe-Cl or it just went bad with time. Not that I've seen that happen though.
Mike