The NC Laws and Rules say:
a) Any person owning or controlling the property upon which a ground absorption sewage treatment and disposal system is installed shall be responsible for the following items regarding the maintenance of the system:
(2) Ground absorption sewage treatment and disposal systems shall be checked, and the contents of the septic tank removed, periodically from all compartments, to ensure proper operation of the system. The contents shall be pumped whenever the solids level is found to be more than 1/3 of the liquid depth in any compartment.15A NCAC 18A .1961 MAINTENANCE OF SEWAGE SYSTEMS
This is not very useful for the average homeowner, so the general “rule of thumb” is every 3-5 years (and we are seeing more and more permits where the county is writing “pump every 3-5 years”).
If a tank has gone 10 years or more without being pumped, a standard pumping may not be possible. The contents often harden up so that they are no longer “flowable solids”. Imagine have a bathtub that is filled with one big block of clay and trying to vacuum it up with a shop vac – it doesn’t work.