We used to run a KVM from our server rack to a desk on the other side
of the room via a 25' VGA and PS/2 cables with no issues, keep in mind
this was only going to a bunch of servers so resolution was limited to
1024x768. You might see some degradation with higher resolutions if
you're using VGA. For video signal you may get better results with
DVI. (See below)

USB cables are also limited in length. USB 1.x is limited to 3 meters
between hubs, and 2.0 is limited to 5 meters between hubs. Both permit
a maximum of 5 hubs connected in series.

Here's what Wikipedia has to say about DVI and cable length:
The maximum length of DVI cables is not included in the specification
since it is dependent on bandwidth requirements (the resolution of the
image being transmitted). In general, cable lengths up to 4.5 m (15
ft) will work for displays at resolutions of 1920 x 1200. This
resolution will work even up to 10 m (33 ft) if a special cable is
used. Cable lengths up to 15 m (50 ft) can be used with displays at
resolutions up to 1280 x 1024. For longer distances, the use of a DVI
booster is recommended to mitigate signal degradation. DVI boosters
may or may not use an external power supply.

-- 
Andrew S. Zbikowski | http://andy.zibnet.us
IT Outhouse Blog Thing | http://www.itouthouse.com