It's rider specific.
Look at the downhill sections of the Tour for example, where the riders are doing 30m/s or more. Or gravity bike racing, where the offroad riders regularly hit 30m/s (60mph) and the record is 130mph, and interestingly the on-road riders are slower, probably because of a lack of suitably steep paved roads. As a comparison point, the UCI record for an upright bicycle is about 20 m/s (45mph).
Personally I don't go much above 15m/s on my uprights and never above 20m/s (about 45mph). But with more wheels and a safer riding position I've felt fairly safe doing a 10 minute descent between 20 and 30m/s. I don't know what my maximum speed was there as my speedo loses the peak speed at ~10m/s and gives a maximum of 146km/hr (presumably related to wheel size and minimum possible time interval on the sensor).