This is a complex subject, and one which Randy Oliver has studied quite carefully.
Rusty Burlew has written a nice summary of Randy’s findings on her web site:
I am inclined to agree with her interpretation. The data seems to show that you need to do it weekly for it to be really effective. You need to use very dry powdered sugar, otherwise it doesn’t work well. You need to be aware of additives in the sugar. Some anti-caking agents seem to be toxic to bees, but corn starch (corn flour) is fine. It works best when the colony is broodless. You need to use a screened bottom board and leave the slider out for at least 24 hours after each dusting, so that the mites fall out of the hive - sugar doesn’t kill them, and if you have a solid floor, they will just find another host.
I think 5 times per year is insufficient, and you will risk losing the hive if you have a heavy infestation. I would rather use something which is more reliable and doesn’t have so many caveats. Oxalic acid would be my first choice, given that it is an organic product.