The max recommended interval for industrial given in GN3 is 3 years, however this is highly dependent on the nature and type of installation, and where other statutory/licensing regs may take precedence, or due to insurance requirements.
the interval may also be shortened or lengthened depending on what the inspector finds, also where regular maintenance schedules and records are kept then it may not be required at all.
Some factories do (a different) 20% per year, so over 5 years everything is covered.
There is no BS as such.