How Often Should I Waxoyl?

The place the wife goes to to get her FTO properly done does it once fully then just touches up over the following two years and a full job the year after that so I'd say the full monty every third year.

I've always understood Waxoyl to be intended for un-exposed areas like the inner surfaces of chassis members, sills, door panels etc, in which case it should never need repeating.

It does a pretty good job on exposed surfaces too; I did my subframe with it when I repaired it. My 1971 car has a lot of areas coated with a wax product, which explains why it has survived so well. I won't use underseal, I'd rather maintain a good paint coat with perhaps some help from a waxoyl-type product (there are other brands such as Dinitrol which have products intended for exposed areas).

I've coated exposed areas with Waxoyl then blown it over with normal underseal.

You can get Waxoyl/underseal mix in the Schutz cans. For heavily exposed areas you can get brush on underseal and Waxoyl mix.