The range of ANT is determined by many factors: the ANT chip you are using, the RF power setting, antenna design, noisy RF environments, enclosure design, PCB noise, etc.
Under ideal conditions and 0 dBm ANT has been shown to work at up to 30 meters line of sight. This drops as other factors above come into play. You also need to be aware that the frequency of ANT does not pass through objects very well.
At -20 dBm you can expect a significant drop of range, down to a few meters under ideal conditions, but range is really something that every developer needs to test themselves, under their own specific hardware and environmental conditions.
