If you are heavy-handed, you can wreck your strings in a few days to a fortnight. Even putting a replacement one on the wrong way may damage a new string! Buy expensive strings and treat them sensibly and you are right for six months or more. You need to change ALL the full set of strings every six months anyway to maintain the best quality of sound. Old strings lose their "sweetness" and response. Wire-wound gut or plastic strings aren't as resilient as steel strings and last a little longer.