The best way to determine if a tank needs to be pumped is to open the cover, and measure the depth of solids using a wooden probe that you plan to throw away. If the solids are less than 15 percent of the depth of liquid in the tank, no need to pump. If more than 15 percent, time to pump. By the way, there is good evidence that a properly managed system can go upwards of 20 years with no need to pump. Specifically, no garbage grinder, don't toss bulky vegetables down the drain (compost them), use limited amounts of laundry detergent, you may never develop large amounts of solids.
You should routinely pump, though, so the tank, baffles and inlet can be checked for fractures and other issues. A system can be relatively low on sludge but still have other issues that need to be addressed.
At this present moment in time I am making cabinets for the kitchen - just in case you wanted to know what I'm doing when I'm not around.