Translating 16-bit digital counts on a CD to voltage on the output RCA connector
There isn't a strict standard, but "nominal" line level for consumer-grade gear is typically -10 dBu, which is the voltage that would dissipate 0.1 mW in a 600 &Omega; load. This works out to about 0.245 Vrms, or 0.346 Vpeak.

Now, the level you are asking about is relative to the full-scale range of the DAC. The key question is: What is the relationship between dBFS and dBu? This depends on how much "headroom" the designer of the CD player decided to build into the DAC. A typical relationship is that full-scale is 10 dB above the nominal level, which means that the clipping level of the DAC (full scale) is 1.095 Vpeak (0.775 Vrms, or 0 dBu).

Therefore, a -6 dBFS signal (half the voltage) would be 0.549 Vpeak, or 0.388 Vrms. YMMV