If you really are religious about only putting human waste and TP into your system, you can almost always go for at least 5 years without pumping.

If you know where the tank cleanout cover is located, you can did it up and stick a rough-surfaced wooden pole into the tank, If there is sludge build-up, you can see it when you remove the stick. If the stick is clean, you are in good shape and not in danger of clogging the drain field pipes.