In order to change task duration you need to go to Tools->Configuration. Here you need to open the object ActivityUtils. What you're looking for is the following line "var dueDate = startDate.AddMinutes(30)". The next step is to change the value "30" to "15" and publish.
Hello. Is this still working in newer versions (7.18.x)? I've been searching for it and I didn't find it. Thank you!