We had ours pumped a couple years ago, and I asked the guy how often we should pump- he said for family with kids, every 5 years, and for just a couple, 8 years or so. Of course, there's probably a little self-interest on his part in that recommendation, and you also have gray water diverted.
But you should do it now, because ours had a deteriorating standpipe (which keeps solids out of the drainfield), which he fixed. The problem is that once you see trouble in the drainfield, its too late, because you can't magically get the solids out- you have to dig a new drainfield. That can be especially troublesome these days, because everything has to be permitted, and the regs are much stricter now. Even if a gravity system would work fine, they'd probably make you put in some kind of mound or pre$$ure $y$tem, if you get my drift.