I am not sure I understand your scenario correctly, but I suppose that showing and hiding the RadAjaxLoadingPanel manually may be more convenient in your case:
The above example uses the RadAjaxManager's OnRequestStart and OnResponseEnd events, however, you can use any events to do the same.
On the other hand, you can set the MinDisplayTime property at runtime just like any other property of any other control in any of the available page lifecycle events - there are no additional "special" opportunities with regard to the RadAjaxLoadingPanel.
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