I'm no expert on this subject (I don't know sh!t) but when we had a drain blockage I got the local company out and they did a full inspection while they were here as I didn't know when the tank had last been emptied - and I assumed it was something that had to be done.
They told me that our tank was fine and unless we started to notice a smell for more than a few days we shouldn't need to empty it. The system, as I understand it, is supposed to work partly as filter and allow water to overflow and soak-away, while solid matter is digested and breaks down over time.
The only time we ever have any issues (other than that one blockage) is when we've had a week or so of dry, warm weather, and then I just make a point of running a bit of extra water down the drains.
I know you're not supposed to put bleaches and some other chemicals into the system, but we do put a small amount in when cleaning the toilet etc, and I've not seen any evidence that it's killed off the bacteria.
In fairness, our house was designed as a 4 adult occupancy and there's only 2 of us here, so that probably makes a big difference.