To pump or not to pump, that is the question.
I would have someone from a septic service company open it up and see what you have for solids. If the solids are anywhere near the outlet to the leaching field, you need to pump.
If there are very little solids, that is the sign of a well-balanced system. If that is the case, pumping could actually upset the balance. It is all about what you put into the tank. If it is mainly waste and water, the balance should be good. If it also services the washing machine, it comes down to what is being used for detergents. Detergents can mess up the bacteria and enzymes that need to be in the tank to eat away at the solids. Tanks servicing the washing machine usually need to be pumped much mire frequently that those just handling waste with some dishwater.
Time is only one variable and a well balanced septic system will run a very long time without being pumped.
Good luck!
R2B