the OSHA guidlines in the U.S. RECOMMEND re-training for site specific training every three years for operators and every five for a train the trainer instructor.

These are only recommendations. When I went through my schooling in an actual OSHA taught class, not an OSHA approved class with a random instructor. The trainer told me at that time there was nothing official in black and white for re-certification. This goes back about 12 years ago. You can go to OSHA's web site to see if it has been updated, I havent instructed in a few years so I havent kept up with it.