
A healthy septic tank with a good outflow will look after itself and will not need emptying that often. I've lived with one for the last 22 years, prior to that we had mains drainage, so, like you, we were a little apprehensive initially. We have ours emptied on an annual basis as we do not have a good outflow (under the driveway of the house and on a slope down to the road), however the septic tank man usually reports that all is well with the contents. :-)
I use non biological washing powder as I have been told that biological stuff kills off the good bugs which make the tank work properly and break down the contents. All the house water drains into the tank, but not rain drainage from the roof etc. We have only had one problem with the tank which necessitated a bit of rodding and a man with a high pressure hose.
Don't worry about a septic tank unduly, I tend to forget we have one, just very careful about what goes down the drains and the loo, especially sanitary products.