I always thought it was an annual thing too...but my vet has other ideas: my mare just got her teeth done 24 months post-last float. She actually did not have any hooks or any "issues" besides a small amount of uneven wear.
She is 28 and a very slightly loose tooth was discovered yesterday, during her float, so her next float is scheduled to be in 18 months but will be moved up if need be.
If I weren't a poor college student, I probably would just have it done every year, no matter what, but as it is, floats are expensive stuff!
Apparently my vet's thinking is that younger horses get yearly floats but older ones ideally get 24-month floats with the goal of preserving as much chewing surface as possible as the horse ages. I'm not sure how much I buy that idea, that it actually works, but I know she wouldn't refuse to float Lacey if Lacey needed a float in a year vs 2 so she can have whatever theory she wants. After all, she IS the vet! Haha
Fabio - 13 year old Arab/QH gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.