We just moved into my mother in laws house, and had some problems with the septic tank. There is an apartment above the garage as well, so there is a total of 3 bathrooms, and two families doing laundry, which is a lot of laundry, and dishes, and showering. Anyway, the toilet in the house would overflow on occasion, and water would back up into the bathtub of the downstairs bathroom. We eventually found out that the septic tank was full, and had it pumped. This was around the end of January beginning of February, and it is a 1300 gallon tank. It is now only June, and again the toilet has overflowed, and water is backing up in the tub. If I try to do laundry, I can hear the water bubbling in the sink. It isn't possible that the septic tank could be full again in just 4 1/2 months, so what could be causing this to happen again?

These are signs of a system falilure, maybe there is something wrong with the drainage field which is causing the tank to fill up and not drain properly. You should have a plumber out to look at it.

Septic tanks are actually almost full to the top all of the time. Gray water does not flow to the leach field until the tank is about 4" from the top. Around here your tank would be rated for 4 bedrooms which means a max of 8 people. Who knows the size of your field but I agree with amricca, this is a sign of failure of the field over over use by too many people. A plumber may not be your best start since most septic fields are build by sewer contractos, check the yellow pages.

Thank you for your helpful answers... It does make sense and was actually something I had wondered about...
There are actually only 8 people total living here, house and apartment included, but the apartment is occupied by my 19 year old daughter and her 21 year old boyfriend, and somehow, they do as much laundry as I do with 3 young children, And, a 17 year old stepdaughter who does her own laundry when she needs to.
Another thing to consider now that the leach field has been mentioned, the area where it is located is always wet, but this is due to a runoff from the hill above the back of the house, and there is also a pond between the two, but right now the drain is raised. I'm guessing the area most always being wet doesn't help. At least I can show this information to my husband now, and we can take steps to fix it. Thank you so much!

That damp leach field is what's killing you. If they are wet the field has failed or overfilled or it never passed a perc test. A family your size can fill a tank in a week and then the field must handle over 1000 gallons of water a week there after.