Idk, if the symptoms will disappear or not. It says the disease can be acute or chronic. Since they didn't get the proper dose the first time, it's possible the bacterium could have developed some resistance already? Not sure how fast resistance develops. However, there are other meds you can try if the Tylan doesn't work. Read this link again.
http://www.thepoultrysite.com/diseaseinfo/82/infectious-coryza
Meds are listed that you can try, here's one that's different: http://www.randallburkey.com/Sulfadimethoxine-107-GM/productinfo/26045/, and if there is a state agency that does tests or maybe even a university it looks like a nasal swab could be used to test for Coryza if you want to be sure.
Also, one other thing I would suggest is a sanitizer for your water. Many infections are spread by the community water trough. Little birds also come to drink from the water trough. So, if you want to prevent the spread of disease through the oral route you may try using a sanitizer in the water. I recommend Oxine. Don't mix it with the citric acid, just add 15 drops per gallon of water and it kills many bacterias, viruses and algeas etc. Now, the chickens may take some time to become use to it, so you may have to slowly adjust the dosage for them to accustom them to the taste. If you ever plan on hatching chicks from your birds ( I wouldn't recommend that if they are carriers though) I would withdraw the Oxine because I have personally witnessed what I believe to be toxicity problems with a few hatching chicks from the stuff. Don't mix it when using antibiotics or vit/electrolytes in water either. But it can help stop the spread of disease from the oral route. It can also be used as a sanitizer for your coop and equipment if mixed at a different dose and sprayed on stuff after regular cleaning. You can find it online, but it is sold by the gal and is a little pricey.
