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I think you should keep them as short a possible. Can someone confirm if this is true?

Some considerations:

If the CT's are located too far away from the emonTx then you should consider loss in the current measured and interference from other electromagnetic sources like other high current conductors in the vicinity.

Depending on the distance you need you can try to use a very good quality shielded wires, like coaxial cables.

Since the impedance presented by the emonTx is very low (the burden resistor mainly, which is around  20 to 50 ohms) the type of cable wont`t make substantial differences, just use a good one but keep them as short as possible anyway.

Hope this helps.

Two points.

1. You cannot lose current in the conductors. If the voltage drop is too great, then the c.t. will saturate. However, it would need to be a very long and very thin cable before this became significant, as the current is only 50 mA  maximum using the standard current transformer.
2. R.F. co-axial cable is a bad choice. You should use twin-core screened microphone cable with the 2 cores twisted. You should earth the screen at one end, preferably the c.t end, and connect the two inner cores to the c.t. and to the current input

The main problem is likely to be interference from other cables and switches. Avoid running the cable parallel to any other cable unless at least 1 m separates the two cables, if it must cross, cross at 90°.

A cable like this http://uk.rs-online.com/web/p/multicore-microphone-instrument-cable/2074... should be suitable.

You should keep the cable as short as possible.

so if there are other cables near to interference?

how far you have to find??