What is the maximum length allowable for a single USB cable?

Per the USB specifications, the maximum cable length from a USB port (or USB hub) to a USB peripheral device is as follows:

For a Low Speed (USB 1.0) device:  3 meters (about 10 feet)
For Full Speed (USB 1.1) and High Speed (USB 2.0) devices:  5 meters (about 16 feet)

General notes:

• Self-Powered USB hubs are generally recommended over Bus-Powered USB hubs, primarily depending on the attached USB devices (how many amps they require).  Self-Powered hubs use a power adapter and output 5 volts / 500 milliamps to each USB port, whereas Bus-Powered hubs get power from the upstream USB port and output 5 volts / 100 milliamps to each USB port.  All Digi Hubports are Self-Powered.
• USB "active extension cables" are not recommended and unsupported.  Consider using a Digi AnywhereUSB network-attached USB hub for applications that require a USB device to be further away from the computer than USB spec allows.

Notes specific to Digi products:

• All Digi Edgeport USB-to-serial converters are USB 1.1 devices.
• Digi Watchport USB cameras and sensors are USB 1.1 devices.
• The Edgeport/416 has a built-in 4-port USB 1.1 hub.
• Legacy Hubport models are USB 1.1 hubs and current "/c" (compact) Hubport models are USB 2.0 hubs.
• The legacy AnywhereUSB/5 is a USB 1.1 hub.  Current model AnywhereUSBs are USB 2.0 hubs (with newer firmware, otherwise USB 1.1).