RadBusyIndicator control supports two main modes of execution. It can be either determined or indetermined. These correspond to the following scenarios:
a specific period of time
an indetermined amount of time
The default scenario is the second one - the RadBusyIndicator is indetermined. You can control this via the IsIndeterminated boolean property. Its default value is True.
If you need a determined RadBusyIndicator you have to set the value of the IsIndeterminated property to False. In this case you can modify the ProgressValue property, which will indicate how much of the predefined time have already elapsed. You can set its value through XAML or code-behind. However, to get the most out of it, you have to bind it to a percentage value (between 0 and 100) indicating the state of the ongoing process.
Read more about reporting the current stage of loading via the ProgressValue property here.
Here is an example of creating a determined RadBusyIndicator control:
<telerik:RadBusyIndicator IsIndeterminate="False" />
How much of the inner donut is filled depends on the ProgressValue property. If it is less or equal to 0 the donut will be empty and when it is grater or equal to 100 the donut will be filled.