My water softener can display average daily water usage. We (family of 5) use about 160 gallons per day. We'd fill that tank in less than 10 days.
How often to pump varies. My tank hasn't been pumped in 5yrs- works fineYour tank should be pumped every 2 years. The "T" they installed should be installed on the outlet side. Your drain line needs to slope a 1/4" per foot. Sounds like your drain line has a slope problem or roots. Have a drain cleaner come out clear the line & put a camera thru the line to check for problems. Make sure he gives you a copy of the video (no video no pay), or you can buy a power snake & snake the line out everytime you have a problem. You should have a cleanout inside the house. If not remove the basement toilet & snake from there.
Suggested pumping frequency is somewhat location/climate dependent; here in AK the temperatures are so low that there is little or no bacterial action to reduce the volume of solids. For a typical household, annual pumping is recommended.The topic of how often a septic tank should be emptied has been discussed repeatedly on this forum..............The most authoritative studies I located indicated that a carefully managed system could go ten or more years between pumping with no issues, and a poorly managed system may require pumping every year.