Our pump service is for up to 1,000 gallons.
Most tanks in this area are 1,000 gallon tanks. This means the tank holds about 1,000 gallons of wastewater and still have 9 inches of air space between the top of the waste and the top of the tank.
A tank that is overly full or backing up into the house may have more than 1,000 gallons in it. One pump may not completely empty it but it will get most of it and (for a system working and being used properly), pumping again in 2 or 3 years should be fine.
1,500 gallon tanks are not uncommon in this area – again an 1,000 gallon pump will get most of it, but if you want it completely empty, you would need to order a second pump.