If it has been done correctly then not very often. I've seen box sections that have had anti rust wax injected into them that are still showing good coverage with no shrinkage 8 years later.
I do my classics every three years or so, or (obviously) whenever the wax coating has been disturbed. Like you, I use Waxoyl as that's what I've used for 25 years with no problems, but there are those who say that there are better products on the market.
