When does differential signalling become necessary
In general, when using long cables, the grounds a both ends could be different.
And with either long cables or high frequencies, the signals are more susceptible to noise.

In practice, the limits depend on the actual noise levels.

TI's application note Comparing Bus Solutions mentions limits, which are appropriate for the environments these interfaces were designed for:


  
  
  The speed and distances listed are estimated maximums and simultaneous
  operation at both maximums may not be possible. As cable length increases, you must lower the signaling rate to keep the same bit-error rates. This, and the increased exposure to noise with distance, makes the analog characteristics of line drivers and receivers important and helps explain the number of different standards. It also complicates the selection process.