How fast can wln_scan execute?

I want to scan for available networks almost continuously and hence I have put the following command in a while(1) loop:
wln_scan (NULL, 0, WLN_CHAN_SCAN);
I have set up an event handler
wln_set_event_cb (event_handler, NULL);
Each time it should generate
case WLN_EVT_SCAN_RESULTS:
at which my code takes necessary action.
This is based on the wln_scan application provided by Digi.

However I have seen that if there is no delay inserted between consecutive wln_scan commands, at times event returns with 0 networks found when infact there are networks present around. Is there a minimum delay needed between two wln_scans?