The item you want is Total Dissolved Solids from the water quality report http://www.irwd.com/images/pdf/services/IRWD-WQR.pdf. It's 2nd to the last in the 3rd grouping of data called secondary measures. Hasn't really changed much over the last 20 years.
Back when I ran a deionizer for other reasons, I regularly saw effective rate of about 700 PPM TDS.
Then if you take a system like https://crspotless.com/spotless-water-facts/ look at the 10 inch model, you basically can say you'll get 1/3rd of the output of the 200 TDS level. (since we're running 600 TDS) or about 70 gallons. The big 20 inch model will probably get you around 125 gallons before needing to replace the filters.
That pretty much matches my experience with different deionizers. The filter get eaten about 3X as fast as their "national" average rates.
I saw a lot of seasonal variability too. Spring water was softer, Sept/Oct liquid rock.
