The standards for CAT 5e and CAT 6 call for a maximum run length of 90
meters or 295 feet. While you may make a longer run and get it to work, this
is not recommended. Even if problems don’t show up immediately, they may
come up later as your computers are upgraded to transmit faster or your
network is expanded. If you must make a longer run, you can either put a
signal repeater in the middle of the line or you can make your long run with
fiber optic cable.

If your run is between 295 and 590 feet, the repeater option may be the best
option for you. A switch or hub will act as a repeater. Make sure that you
locate it in such a way that both runs on either side of the repeater are
within the 90 meter standard. It won’t do much good to split a 500 foot run
into two runs of 100 and 400 feet.

If your run is longer than 590 feet or if most of the run is outside, fiber
optic may be your best option. You’ll need a media converter or fiber fed
switch at each end of the fiber run to convert your network signals back and
forth between electrical signals on copper and light pulses on fiber. If you
are running fiber optic cable underground, you’ll need to protect the
delicate glass fibers inside the jacket by running it through a conduit or
by using armored fiber optic cable.

UPDATE: Here’s some clarification on the 90 meter maximum run length. This
is generally cited as the maximum horizontal run length. The max length from
switch to node is 100 meters. This 100 meter length, however, includes the
vertical drops and all passive connections such as the patch cables from the
switch to the patch panel and the patch cables from the jacks to the
computers (or other types of nodes). Big thanks to Marc for
calling the need for this clarification to my attention.

[Bethany Hunt of FOURPAIR]