NFPA 101 stipulates once a month for educational facilities, which is what most states end up going with. But, some states adopt that standard with changes, making required drills more frequent depending on the AHJ.
That’s the brass tacks of it all, but if you want to know why the standard stipulates that, it’s because of tragedies like the Our Lady of the Angels School fire. Unfortunately, changes to the code are often reactive rather than proactive.