You can apply as often as you'd like! This product can be layered multiple times. Just be sure to wait 6 hours in between coats to let them it properly bond and cure.
When applied correctly you will get up to 1 year of durability and protection depending on your washing regime and climate, but for the absolute best results and for daily drivers we like to apply every 6 months; once before winter and once before summer.
Water beading will last around 6-8 months, again depending on your washing regime and climate. So you can certainly get away with applying once a year, but we like keeping our vehicles looking fresh!