Most states have info available that is pretty good. Septic tanks have been around a looooong time and are pretty well figured out.
Oregon DEQ:
Oregon DEQ septic tanks Handy sheet from WV:
http://www.nesc.wvu.edu/pdf/ww/septic/pl_fall04.pdfFind a good pumping service that will check about every 3-5 yrs (with only two of you you can probably go 5-7 yrs), pump as needed.