You can show change the timeframe on your profile.  Log onto your account and go to ACCOUNT/DETAILS and scroll to the SETTINGS box at the bottom of the page.  Go to CALLBACK TIME and select your desired time from the dropdown menu.  Selecting a shorter timeframe will increase the chance of the customer waiting for your call – if a customer is told someone will call them within 15mins, they are more likely to wait that if they are told someone will call within 2 hours.

But if you are changing this, be sure you are able to adhere to your promise.