user1612: I am building a project in which two Arduinos will be linked by three wires: TX an RX for serial communication and a common GND. What would be the maximum length of the wires here before communication gets affected?
I am using an Arduino Mega R3 on one side and an Arduino Mini on the other one.
I have no particular requirement for the Baud rate, I could use the lowest one if this helps in any way.

user6959: 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. 

1612: <p>I am building a project in which two Arduinos will be linked by three wires: TX an RX for serial communication and a common GND. What would be the maximum length of the wires here before communication gets affected?</p>

<p>I am using an Arduino Mega R3 on one side and an Arduino Mini on the other one.</p>

<p>I have no particular requirement for the Baud rate, I could use the lowest one if this helps in any way.</p>
 6959: <p>You need to specify </p>

<ul>
<li><p>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.</p></li>
<li><p>How you intend to connect to the circuit - direct Arduino pins, high/low buffer, differential drivers, ...</p></li>
</ul>

<p>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. </p>

<p>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.</p>

<p>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. </p>
