The average reverse osmosis waste water ratio is 4:1. This means for every gallon of clean water, an RO system can use or waste 3-25 gallons of water.
Considering a running tap puts out anywhere from 1-3 gallons a minute, you’re looking at 9-75 gallons of wasted water for every minute you spend washing dishes.
That said, a well-designed reverse osmosis system doesn’t waste water so much as it uses it. Fortunately, as RO systems become more advanced, you’ll find more and more options for systems that use significantly less water.