I've used active repeater USB cables to get runs of 35 feet PLUS the 15 foot max on the "regular" USB cable. That's 50 feet in total. Naturally, this might not work for every possible device in the world, but I've had no problems in my studio (AxeFX <-> computer connection), office (computer <-> web camera), and home theater (XBOX <-> Kinect sensor) with runs this length. I configured a customer's pedalboard for his AxeFX <-> McMillen SoftStep controller with a 50 foot USB interconnect and it's worked for him for years.
With USB 2.x gear, the biggest challenges in longer USB runs are voltage drop and RF interference. A USB connection supplies 5v DC (+/- .25 v), which drops over distance. Some devices are particularly sensitive when undersupplied. I haven't had many problems using active cables, at least in terms of the voltage provided at the terminal connection. On my multimeter, the power is still within spec even at 50 feet. Some cables - especially lower cost active cables with crummy shielding - can turn the long cable into an antenna, which negatively impacts data communication. The USB 3.x / type C standard adds additional RF interference suppression to the data channel, which should help with longer runs. At any rate, in my experience, a well-made active USB extension cable is reliable up to this distance.
For the record : I agree with M@, that for anything critical, keeping the length to 2 meters is a great policy for USB 2.x.
