You are able to choose between 15 mins, 20 mins, 30 mins and 1 hour as the time granularity used on the scheduler. The default selection is 30 minutes. For simplicity and user-experience, you should choose the largest time granularity that satisfies your needs.
You can change the value by going to Settings > Venue Profile and choosing the Basics section.
This contains a section called 'Culture, Currency & Time', that will allow you to set the 'Time granularity'.