Target response times vary from business to business, but the answer should always be as soon as you can.
Some organisations publish their target response times, and five or ten working days are common timescales – depending on the nature of the business. Whatever number you choose, be ambitious. Ask yourself if you can do better – at least some of the time. If you’re a smaller company with fewer customers, your response time should rather be measured in hours than days.
Of course, some complaints are genuinely complex and take time to resolve properly. But remember, every day a customer is left waiting for a response is day that they could visit your competitor or tell their friends about their bad experience.