I actually went 14 years (!) without getting the septic pumped -- it was overdue to be sure. More recently went 5 years.
I did a large addition to the house in 2010, adding a bathroom (taking the count to three) but not a bedroom (my setup is three bedrooms + disposal but only two persons) and the county insisted upon adding a second septic tank for the addition
but without a leach field. Instead, they specified a line that pumps from the new tank to the old tank, thereby integrating the two tanks. Talk about $$$...