What is the maximum wire length for serial communication between two Arduinos?
You need to specify 


The order of range you are interested in - is it in the order of metres, tens of metres, kilometers or interstate ?:-) . ALL are possible with suitable compromise.
How you intend to connect to the circuit - direct Arduino pins, high/low buffer, differential drivers, ...


If you use 5V/0V "unipolar" then at say 9600 baud you can get many metres with suitable drivers. If you use an Arduino pin directly wire resistance and inductance will start to matter as range increases. 

At 300 baud you can achiever hundreds of metres with twisted pair cables. As range increases aspects like cross-talk from the other circuit and external noise increase in importance. At longer ranges having 4 wires with signal + ground twisted together in each case helps BUT it is likely that at ranges of interest 2 wires plus ground will work OK.

If you are prepared to use enough voltage and slow enough signalling you can signal over thousands of miles - as was done with very early telegraph cables. The thousands of volts needed and signalling speeds of perhaps a few bits per second will be a disincentive in most cases.