5 meters is the maximum USB cable. You can get further by connecting USB hubs serially (maximum of 5 hubs chained this way). You can't just use USB extension cables due to timing issues.
From the USB FAQ:
Q1: How long of a cable can I use
to connect my device?
A1: 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).
Q2: Why can't I use a cable longer
than 3 or 5m?
A2: USB's electrical
design doesn't allow it. When USB was
designed, a decision was made to
handle the propagation of
electromagnetic fields on USB data
lines in a way that limited the
maximum length of a USB cable to
something in the range of 4m. This
method has a number of advantages and,
since USB is intended for a desktop
environment, the range limitations
were deemed acceptable. If you're
familiar with transmission line theory
and want more detail on this topic,
take a look at the USB signals section
of the developers FAQ.
Q3: How far away from a PC can I put a
USB device?
A3: With the maximum of 5
hubs connected with 5m cables and a 5m
cable going to your full speed device,
this will give you 30m of cable (see
section 7.1.19 for details). With a
low speed device, you will be able to
get a range up to 27m, depending on
how long the device's cable is. With a
straightforward cable route, you will
probably be able to reach out 25m or
so from the PC.
Q4: I need to put a USB device X
distance from my PC. What do I do?
A4:
If X is less than 25m or so (see
previous question), buy a bunch of
hubs and connect them serially with 5m
cables. If you need to go farther than
that, put another PC, or maybe a
laptop, out where you need the device
to be and network it with the first PC
using something that's intended to be
a long-range connection, such as
Ethernet or RS-485. If you need to use
nothing but USB, consider using USB
based Ethernet adapters to hook the
PCs together.
