At what cable lengths are termination resistors required for RS-485 networks?
In general, for short cables (&lt; 20-30m) and low baudrates (&lt; 115200) you can leave them out without much trouble. But:


It is useful to put some kind of load on the signal lines to improve noise immunity (the RS485 driver will supply enough current to switch the voltage on the differential line, many noise sources will not). But you do not need this load to be equal to any "characteristic impedances", \$200-500\,\Omega\$ will be ok.
When you go for high speed or long cabling you will need proper termination that depends on the cable you use. So this should be \$100\,\Omega\$ for Cat 5 cables (not \$120\,\Omega\$).


Don't forget about the pull-up and pull-down resistors. They are required unless all receivers used in the system give a well defined (high-level) output for \$0\,\mathrm{V}\$ input. Their values should be chosen so that (when connected together with the "terminating" resistors) the un-driven line is properly polarized (\$&gt; 0.3\,\mathrm{V}\$ for most receivers)