a week of medication will normally do it, but medicated feed is weaker so I would go 2 weeks. medicated/unmedicated is a big decision for all of us.
although several newbies in the chicken world think that chilling and cocci are the same thing. the symptoms are similar but cocci is much rarer especially if you go ahead and feed the medicated feed through the first 8 weeks of the chick's life. the antibiotics will wear off long before you eat any eggs or meat from them. and if you have a respiratory infection, wouldn't you go and get some antibiotics?
since you haven't been feeding medicated feed, you can use corid. its actually for cattle, but works for chickens too. this will cure it in one week if its cocci. give another half dose 1 week after finishing treatment.
if your birds have been chilled, you can warm them up, keep their butts clean, and add some sugar or raw apple cider vinegar to their water. pasturized will work but not near as well.
sugar helps them get rehydrated, acv helps rehydrate and helps respiratory problems. you can mix the 2 (1 tablespoon each to 1 quart of water) this will perk them up in 24 hours usually, but the butt cleaning will go on a couple of days longer. keep giving it until the butt is clean.
brian