kc
it really depends on what it is initially treated with. if it's someting Thompsons water seal you may need to do it yearly. personally, after researching and on the advice of my tub manufacturer, I used the Sikkens brand. one coat of the cetol 1 and 2 coats of the cetol 23. That was 2 years ago. with a rinse off the sides still look brand new. however, the top rail which on my tub, has exposed wood, could use a new coating this year. another factor is how much direct sunlight your tub receives. the more direct sun the more frequently you will need to treat it.