Notwithstanding all the FTers who got invited to RA in different circumstances, this seems to be the "official" criteria for an invite to RA status - the first time I've seen a it being put in words on an Intercontinental website:

http://berlin.intercontinental.com/berha/offers_02.html


You will be invited to be upgraded to the level of Royal Ambassador when you have spent at least 50 nights a year in three different InterContinental hotels and you will receive even more benefits.