Hello Caitlin: Typical average home water use is as follows: Toilet 24%, Shower 20%, Faucets 19%, Clothes washer 17%, Leaks 12% and other 8%. The exception to this is if lawn irrigation is used. Irrigation can be a significant water use. It is true that a portion of water will enter the groundwater, however if irrigating during high temperatures, there is also a significant portion being evaporated and transpired (exiting plant leaves) that is not recharging the aquifer. There are ways to conserve water. See: https://extension.oregonstate.edu/gardening/lawn/lawn-watering-guide-eastern-oregon

Leaks are often a cause of significant water consumption depending on the type of leak. Check for running toilets and repair these. Leaks in an irrigation system can be an issue also. 

Hello: This is a good question and I have note seen a exact number. However, it is important to note that a septic system is designed for all the wastewater to infiltrate the soil for treatment of organics, nutrients, pathogens, etc. Some water will be taken up by plants, but the majority of water is infiltrating to the groundwater.