In order to change the number of visible days in the TimelineView isn't straightforward. This is controlled via the CurrentScale property of the TimeLineView which is, in this case a DayTimeScale. However, the CurrentScale property does not have a setter.
I have managed with the code below to change the value and it is reflected in the RadScheduler, but it only displays correctly after using the scrollbar to scroll the next section into view.
I will continue to have a look at a way to do this for you, but in the meantime, I thought this code may help you to get started.
dayScale.DisplayedCellsCount = 20
currentScaleProperty = p
Perhaps there is an easier way. I will let you know as soon as possible.
Hope this helps