It's complicated! Whether you need to see someone or need treatment depends on a lot of things. But a normal cough as part of a cold - 4-12 weeks might be ok - but 4-6 weeks coughing should trigger at least a little concern from your GP. The things that can influence treatment and investigation might be age, smoking habits, whether you have other problems or chest disease, whether you are well or breathless. Said it was complicated - probably need to see your GP to make sense of it!