I would probably display a "progress" indicator on the panel, then cause it to disappear when "done".
Exact method depends on exactly what you are returning. Look at the "done" element of the search, and the "rejects" element of the panel. Basically, have something that is listed in the rejects for the panel (for instance "WaitForSearch"), define (set) that something when the search launches and then undefine it (unset) when you reach the "done" for the search.
See the Search Event Handlers section of this page - http://docs.splunk.com/Documentation/Splunk/6.5.1/Viz/EventHandlerReference#Search_event_handlers
See also http://docs.splunk.com/Documentation/Splunk/6.5.1/Viz/tokens#Custom_logic_for_dashboard_eval_expressions
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