This can't be answered without knowing anything about your chameleon, his infection, the antibiotics given and involved bacteria. Antibiotics have different ways to fight bacteria and need different effective levels to work, therefore some need more and some less time to fight an infection. And bacteria differ in favourite tissues and conditions, ways to flee from medication... etc. A sucessfull treatment then depends on how the medication was given (orally, injected, nebulized...), whether keeping conditions or immunsuppression leading to the respiratory infection were changed and whether the infection was quite serious or already treated from the first small signs on. So... unfortunately there is no "xy weeks and everything should be fine". Every chameleon patient is an individual.