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15' is the maximum recommended for USB if you are controlling anything (i.e. a mouse on it etc.); if it is being used to connect an audio source you will have no issues. I have used longer runs when using a MIDI/USB controller but you can run into issues if the drivers aren't solid.
5m is the maximum length guaranteed in the USB specification for devices operating in high speed or full speed and 3m for low speed. The issue you run into is that it actually takes too long for the signal to propagate and depending on the host, it may give up. At 20', you're probably right on the edge, so reliability is not guaranteed. I'd look for a way to arrange your setup so that you can use a 5m cable and ensure a stable connection.
Careful on what you buy. I got a 15 foot off amazon going the cheap route, sure it works fine for little jobs like sending jobs to a printer, but if your doing audio streaming it didn't work for me and I ended up tossing it cause after about 45 minutes the application would lock up using audio streaming.