It depends on the number of people that are on the system. But typically anywhere from every 2-4 years.

If your tank is not full there is something wrong with your tank, this means that it is leaking out somewhere. The average household of 4 people will fill up there tank in 1 week. For your system to work properly it has to be full up to the outlet pipe that goes to the distribution box.

We only dispose at the wastewater treatment facility.

You shouldn't put anything into your drain that will not break down in the tank, (ex. baby wipes, feminine hygiene products, paper towels, extra thick toilet paper, grease, bleach, drain cleaner) harsh chemicals can actually kill the bacteria that makes your system work.

No! garbage disposals are designed to be used in city sewage systems. The bacteria in a septic tank can't break down the waste that goes through a disposal. Your system has to work hard enough with out adding more to it.

Something is wrong with your drain field. If too much sludge gets into the field it creates a film that will not let the water drain into the ground, so instead of going down the water eventually starts to come up into the yard. Your field may need rejuvinated or it could be something as simple as a clog in the line that needs cleared.