Depending on the brand, RO membrane quality, pressure etc the TDS of the water is going to differ a fair bit.
As a general rule, an RO unit working at maximum performance will have an efficiency of about 98%, most important is the water pressure of about 60-80 psi but different units have different requirements. Then there is prefilters, water temperature etc which all affect performance.
I would keep it flushing for a few hours and if needed add a DI unit.
As a comparison, my tap water ranges from 40 to 70 odd TDS and even my RO unit would not bring this down to 0 without a DI unit. I usually get 1-2 TDS after the membrane.