LAN cable: how long is "too long"?

Yes.

100 meters for each run from the router.

Says who?

Cable specifications for the 10-Mbps 10BaseT cable with RJ-45 connector:
Category 3 or Category 5 UTP with 22 to 24 AWG
Maximum segment length ..... 100 m (328 ft.) for 10BaseT
Maximum network length ....2,800 m (9,186 ft.) (with four repeaters)

Cable specifications and connection limits for 100-Mbps transmission, RJ-45 connector.
Category 5, UTP, 22 to 24 AWG
Maximum segment length .... 100m (328 ft.) for 100BaseTX
Maximum network length .... 200 m (656 ft.) (with one repeater)

Of course, it doesn't drop off the earth at 100.1m, so you can get away with pushing the limits of the spec in many cases. Network timing also comes into play for large, long networks, which can increase message collisions and therefore reduce throughput.

Correction
Cat 5e (1 Gigabitps) is 75m actually.
Cat 5/6 is 100m

OK here are the specs straight out of the COMPTIA book as I literally was in the course as I posted

Everything all the way from Cat3 was ALWAYS 4 pairs of twisted Wires
Cat 5 is 100BaseT (100 Mbps w/ 100 m range)
Cat 5e is 1 Gbps (1000 Mbps) w/ a 75m range

Cat 5e ONLY has a 100m range if you get Base-TX. (Not base T)
Cat 6 is 100m and supposedly can do over 1.2 Gbps but...thats the spec.

That's the real, hard specs. No room for interperation or arguement. I'm reading it straight out of the 2006 Comp TIA book.

Ice,
I actually don't know the physical difference between T and TX, the book is too far away to look it up. I do know that it was a test question on our practice test, and the answer was T = 75m and TX=100m. I'm assuming its something as simple as a more pure-copper set of wires, or perhaps a different alloy of metal than just copper. Perhaps it also has thicker rubber for better EMI/signal shielding.

All of these cables use an RJ-45 style ethernet connector and will fit into any Ethernet NIC. A 100Mbps NIC obviously can use them all, but its not going to transmit any faster than 100Mbps so there's no point in using Cat5e or Cat6.

If you have a Gigabit (1000Mbps)NIC, then before you go and get Cat5e or 6 cable, ask yourself what you'll be doing. We all know your broadband net connection will never DREAM of getting up to 100Mbps even. The ONLY instance where Cat5e or Cat6 would benefit a home user is if A) He wanted a LAN of several computers in home B) they ALL have Gigabit NICs C) they were networked together with a Gigabit switch and D) ALL cable used was Cat5e or Cat6. In this scenario, the comps on the network WOULD take advantage of gigabit if they were talking to eachother for info, such as LAN games. But as far as reaching the internet goes, the Gigabit would show no advantage for any of the computers.

The OP didn't make a reference to any specific style of cable or NIC.
OK there's a lesson straight out of the A+ hardware certification course, that'll be $100 for each reader plz. Just PayPal me. :-)

I believe you mean there is a difference between data and experience. As data and knowledge are the same thing. All I did was post the data. The data is correct. I didn't say who is right, who is wrong, what NIC's use what cable, etc. I just posted data straight from the book. Take it or leave it, no sense in arguing fact, Mr. Ice. I don't care if most NIC's use TX...Maybe they do, maybe they don't. I just posted what the range of different types of cables are. Seeing as how that is what the OP asked (Maybe he wants 5e, maybe he doesn't) that is what I answered.

Whatever Mr. Ice, I believe both of your recent posts directed towards me were rather snotty sounding...and I'm not quite sure why. Who knows it could be something as silly as a subconscious aggression that only a pyschiatrist could explain. Either way..The OP wanted to know the range of the cable. Since he didn't have a specific one posted, I gave a few different examples. All examples are fact, nothing else. So, in fact, I believe my post answered the OP 100%.

Ok that's the last look I'll take at this thread, Mr. Ice.

Dear 'I',
Read your chat with Mr. Wolfe. We ran a Cat5 from one building to another, about 550-575', not aware of the length restrictions. Wireless is not an option 'I', as there is too much noise. (Transformers)
'I', can you recomend any type of signal booster to get us up, or are we F'd, and forced to replace it with Cat6?
I appreciate your input 'I'.
Thank you.