RO permeate (RO water) quality is dependent on the salt rejection rate of the RO membranes and the TDS of the feedwater. Many RO membranes have a 98% salt rejection rate. This means if the feedwater has 100ppm TDS, the permeate will have 2ppm. However, if the feedwater has 10,000ppm TDS (brackish water), then the permeate will have 200ppm TDS. Ocean water averages 35,000ppm TDS.
So the quick answer is, it depends. Not all RO water is created equal.
I would say if your TDS meter is showing <50ppm TDS you should be fine. Most likely it will show quite a bit less than 50ppm, probably less than 20ppm.
In water quality of the type used in brewing, ppm = mg/L.