Let me put it this way, you either pump the tank before the leach field plugs up and you have a problem, or you pump the tank after the backup, bring in a backhoe, a few truckloads of stone, some new leach line, the county health inspector, and bury a whole bunch of money.
We got a whole load of city folks around here who want sewers installed cause thyre too stupid to pump a septic tank every few years. I pump mine every 4 years with 2 people living in the house, and have never had a backup. I rebuilt the leach field after 30 years, the original was clay tile, and had eventually went bad mostly due to frost. Most folks only get them pumped when they are selling the house, so the buyer won't know the system is screwed for a few weeks after the closing. The new owner then gets to put in a new leach field.