OP I cannot find anything information about never having to pump the DBox. What I did find is what can happen if you don't.
A concrete D-box can typically last up to 20 years. However, in the event of sludge buildup, tree root invasions, improper maintenance, heavy machinery on top of the box or other issues that can cause unrepairable damage to your distribution box, you may need to replace it. Some signs of a failed box could look very similar to a septic tank system blockage or a leach field issue.
https://www.kulksplumbing.com/what-i...tribution-box/
Frequency
The cost to get a septic tank pumped can rise drastically when property owners aren't keeping up with having a tank pumped every three to five years.
Not pumping a tank on schedule will cause solids to build up until a tank's holding capacity is diminished, which results in a clog.
See #4 & #5, in the section 5 Ways to Save on Septic Tank Pumping Costs.
https://www.angi.com/articles/how-mu...mping-cost.htm
BTW, the prices listed on the link above seem high, as a reference my last pumping cost was $240, so shop around. Public sewage in my area is ~$300 yearly vs. my $240/3 years.