Mosquitos take about 14 days to hatch at 70 Farenheit. It depends on the species and temperature. Some species can go from larvae to mosquito in 4 days. As long as you keep a constant harvesting schedule you shouldnt have any problems. It is also quite obvious when mosquito larvae are on their last stage and will get ready to emerse. They are called pupa by that point and just to be sure you should harvest those out as soon as you see them.How do you grow these out and not get overrun with mosquito's at your house? I have a few outdoor tanks, but need to keep a circulation pump going otherwise I get huge amounts of flying mosquito's throughout my backyard. I would love to have plenty of mosquito larva to feed my Angelfish with.
Here is a pic that shows how the pupa looks like. www.mosquito.org/assets/Images/life cycle.jpg
Just be sure that they are mosquito pupa and not other sort of insect larvae as you can get some annoying insects inside your tanks (dragon fly larvae comes to my mind) if you are not sure they are mosquito pupa just discard them.
I keep daphnia backup cultures outside and sometimes I get mosquitos, I just let them be until they get big, before turning into pupa and I harvest them. I net them, rinse them in tap water (my tap water is clean as can be, no chlorines so be careful if yours does) and feed.