But how often is regular for a two-person household?

Most people go about once a day

You already have your answer. Once a year is a small price to protect
the herringbone.

I bought a rural bungalow off a little old lady, who assured me the
drainage system was fine, but had the tank desludged twice a year.

Two weeks later it backed up (and that's as unpleasant as you
imagine).

The pump out man said "Oh yes, the herringbone's wrecked - the old
lady had me round every 2 weeks".

First quote (this was 1999) was £6500 for a complete refit.

Eventually got that down to £2500 for retrofitting the tank as as
processor, environment agency approval, and certified clean outfall
into a river instead of a herringbone.