I used to think it was 30 seconds, but according to ICAO Annex 10, Volume 2, Chapter 5, 10 seconds is correct:
5.1.5 Recommendation.—After a call has been made to the aeronautical station, a period of at least 10 seconds should elapse before a second call is made. This should eliminate unnecessary transmissions while the aeronautical station is getting ready to reply to the initial call.
Interestingly, this only applies to calling aeronautical stations, not aircraft stations.