It mostly depends on your soil. It looks like the University of Florida is recommending 4 to 6 times a year for high-ph soils (like we have in Tamarac):
Trees growing in high-pH or calcareous soils should receive 4 to 6 foliar applications per year of a micronutrient mix which includes zinc and manganese.If you have deep sand, you'll probably be fine with a granular application.

Jeff