Septic Tank Pumping

We moved out to a country place about 3 years ago. We are a family of 8 (2 adults, 6 kids) and this is my first septic system. Two years ago I had the tank pumped out. There was no paperwork, but I found it and uncovered the tank. Based on the diameter and a depth of 4 feet, it looks like a 900 gallon tank.

My question, how often should the tank be pumped out?

I don't mind paying for necessary pumping (about $150) but I don't have a lot of extra $$ laying around for unnecessary pumping. If I ask the pumping places, of course they will say I should do it often.

Is there any checks I can do to see if it's needed? Maybe remove the cover and test the solid level somehow?

I know I don't want the tank to fill up with solids and then get solids into the leach field. Like most kids, ours use toilet paper very freely and I'm sure there is a lot in the tank.

Thanks
Steve

You are correct in your concerns.
Your system is very undersized for a family of your size and you are way overdue for a cleaning.

http://www.inspect-ny.com/septic/tankpump.htm

Part of the design of a septic system is to have room for the waste liquid to have solids settle out of the liquid for 24 hours before the liquid enters the leaching fields. these prevents solids from entering the fields and fouling the bio mat causing failure of the leaching fields.

I would do a lot of reading at the link I provided above as the site is probably one of the best I have ever seen on septic systems and contains a wealth of information. Knowing about your system is key to maintaining its function.

Your septic tank pumper should be able to help you nail down how often your specific situation requires pumping. You probably would do well to start planning on a larger system to be installed.

I like to pump mine when the poo starts flowing down the hill