I have a twin DI setup in my van, I use the customers outside tap to fill up once a day as I live in a flat.
TDS where I work is around 300 ppm and a sack of resin lasts about 6 weeks not a bad return of investment.
Mind you I only earn around £400 - £500 a week if you're like all the other window cleaners on here and earn £400 - £700 per day you might need to buy more resin.
Robert