How far can I run Cat 5e or Cat 6?
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 links 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 at Lockergnome for calling the need for this clarification to my attention.
UPDATE: I have deleted hubs as an option to be used as repeaters. A very helpful email from Steve Anderson of BIP Solutions explains:
Hi, just read the clarification, and thought I should mention that the run length includes hubs as well.
It's to do with the maximum distance between active nodes. Nodes with store and forward will extend the maximum distance.
The limit's imposed by the time it takes for the smallest packet to be entirely on the wire. If a packet is entirely on the wire, and the beginning hasn't been accepted by all other nodes in the same collision domain, then you could have one begin transmitting and corrupting the other packet. The real problem is the original node doesn't know this has happened, so will only retransmit it if requested.