One way to achieve this is with the properties that are explained in the Hours section of our customization demo. If it is not enough for your scenario you can use jQuery to get each element in the hours column and change it as you want:
".rsVerticalHeaderTable th div"
Hope this will be helpful.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed