How Long Does It Take For Post-nasal Drip In A Toddler With Mild Adenoids To Resolve?

My Kid 3 yr has mild adenoids .He is suffering from cold cough on and off last 2 months. Recently I checked his condition with ENT he is saying this is due to allergic cough and post nasal drip is the culprit. This will subside gradually. How long it take for this post nasal drip to cure . He is on Ascoril LS junior curretly. Last 1,5 month he was on Antihistamines, nebulization for almost 1 month and other medicines whatever prescribed by doctors. I reaaly afraid to give any medicine