Most sites I have read suggest 3-5 days. So, the 30mg/kg that Kelly informed of would be 3ml/2.2 lbs,US. According to the Marissa Ames ( http://www.backyardpoultrymag.com/chickens-dont-get-colds/ ) his is mainly to keep away any secondary infection as a lot of respiratory illnesses are viruses...
Although I forgot to mention (because it's been cooler the past couple days) we haven't used it, and always turn it off in the evening either way. Too much of a good thing is never a good thing! :)
I'm not certain where you live, but here in Texas it has been HOT HOT HOT, and everyone around here said it's the HEAT that makes the chickens stop laying. Something we added, to the comfort of our not-yet-laying freeloader Barnevelders, is a fan...they LOVE it. Hope that helps!