Our customers' perception of our service is influenced by whether we meet, exceed, or fall short of their expectations. When we get a chance to help set a customer's expectations, we should be careful to set ourselves up for success.
Let's say a customer asks you to do something that takes 30 minutes for you to do. You probably tell your customer you'll get it done in 30 minutes, right?
This is actually a very dangerous promise with no upside but a potential downside. It takes 30 minutes, so you won't be able to do it sooner and exceed expectations. It might take you 30 minutes exactly, in which case you've simply met your customer's expectations. Even worse, something unexpected might come up and you end up needing an hour to get back to the customer. Yikes!
A better approach is to set expectations that provide you with some wiggle room while still being acceptable to your customer. If you agree to get back to your customer in an hour, you now have a potential upside while limiting the possibility of falling short of expectations. Getting back to the customer after 30 minutes will exceed their expectations. If something comes up and it takes you an hour, you've simply met your customer's expectations and nobody is upset.