In 12 yrs, I've had ours pumped 3 x's--. Every 3 yrs when all the kids were at home. I know that I'm a mean old man, but we had the 4 sheet law as far as TP went & if I ever found anybody putting anything weird in it, my boot would find the appropriate ass.With 3 girls & 2 boys, I think we did pretty good.
I have a grease trap plus septic/leech field. I don't use any additives.