How long can a baby's cough last without concern?

My LO is 8 months old and his cough has now lasted for a little over one week and is still going strong. He was seen last Thursday by his doctor and everything sounded good but I forgot to ask how long this can last normally without it being a cause for concern. No fever but still coughing, mucus and very congested/runny nose. How long can something like this last before I should call the doctor again?

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