I think it is twice a year that every school in my county does ALICE drills. Businesses and churches can schedule police-led ALICE training just about whenever they want. The county SWAT guys do force-on-force in schools, churches, and office buildings an average of once every two months or so and some of those scenarios involve active shooters. All deputies get run through force-on-force active shooter training at a local school once every few years. Not sure about the training schedule for other local departments or the feds.
The training happens a lot, but I don't know how often it lines up closely with a real active shooter.