How long after starting antibiotics for pneumonia can i expect respiratory symptoms to go away (cough, crackling when breathing) in a toddler?

Depends on cause: There are some bacterial pneumonias that respond rather quickly with most symptoms gone within a week or two.There are some non-bacterial pneumonias that can trigger wheezing and cough for several weeks to months and increased tendency for wheezing with any cold.You need to clarify the expected pattern for your kid with his/her doc.