With the NEHerp mix you typically will see larva at 1 week and new flies at 2 weeks. The best way to avoid mites or at least limit them is to toss cultures in the trash at the 30 day mark. We have a sort of system we use with our flies. The cultures we are trying to feed off to throw away that are at the 30 day mark or near to it and move them away from the starting cultures.

When I setup a new culture I use a sharpie marker to write the date the culture was started on it. We always feed from the oldest cultures and work our way through the cultures.

I have not had any issues with mites yet *knock on wood*. There is mite spray you can buy to spray some paper towels and then put the cultures on the paper towels and it is suppose to keep mites away.

I think the more important thing than temps is the humidity. What humidity are you able to maintain in the tank? How often are you having to mist to keep it up? The temps swing will greatly effect your humidity. On all my Dart/Mantella tanks I have over 90% of the top cover with glass and just a small 1 - 3" section of screening for ventilation. In our Azureus tank we actually have zero ventilation and let the air exchange that happens when we open it to mist and feed be all the ventilation they get.