If you recall I had my septic tank cleaned in 2015 and after taking excellent care of it for last six years, I got it cleaned today. I used Wind River Environmental and after finding a coupon online, I paid about $330, which is apparently the going rate in metrowest area of Massachusetts. Everything went smoothly and when the technician tried to sell me a bacteria additive treatment for about $150, I declined, since those additives are sold in home centers for a lot less (you just add it to the toilet water and flush it) and not everyone recommends that they are needed.
The technician did mention that I had extra grease, which surprised me because we are low-fat diet type of people, but may be was just trying to convince me to buy the overpriced additive.
So yes, six years is a longer duration to pump your septic but remember that for a 1,500 gallon tank, it is just two people (no kids for us) and we take extra care of our tank by not throwing any food or fat into the system, and using non-toxic detergents in the laundry and dishwashers. While we use normal amount of toilet paper, we make sure that we do not use the toilet as just a channel to throw whatever is not useful.