Now you can set up schedule CallBack Free widget. See here, how to do this in the settings in the admin panel.
Why do you need the schedule for showing the callback button? A visitor who leaves a request on the site through the widget assumes that he should be called back in the near future. However, users can leave request on weekends and at night, i.e. at a time when you or your manager is not working and cannot process the call back. If a site visitor is not get call within 15 minutes, he is likely to find a solution to the problem elsewhere, and the request will not be relevant. Therefore, set up a schedule for showing the callback button during business hours so site visitors know for sure that they will be called back in the near future and their problem will be solved.