In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).
Practically speaking, you can use a standard USB 2.0 cable up to 20 feet long without the need for signal amplification for most applications. Beyond those lengths, you should really use a USB repeater cable.