Investigate the [customer's] situation. When the investigation will take longer, make a specific commitment to get back to the customer. When making commitments to call back it is important to give a specific time frame rather than telling the customer you will call back "as soon as possible" or "right away." Terms like these mean different things to different people. "Right away" might mean sometime today to you - it could mean within fifteen minutes to your customer.
- Renee Evenson, Customer Service Training 101
Not only is it important to state specifically when you will respond/call back to avoid potential confusion, but also because no matter what time frame you state, you have now made a promise that the customer is going to expect you to come through on. It is these promises that can make or break a service encounter. Be certain to always promise a specific time frame, and always make it realistic so you do not accidentally break your promise and lose the trust of your customer.