If there is maximum cable length which can be used for TENS in Biofeedback scans and occasionally as an extension for TENS and Tubes during treatments. I just want to make my own (I know it is 50 ohms coaxial), little bit longer.

500 cm are no problem. Because of the small amount of current flow that you need, there is not a significant ohmic loss in the cables.

Another point is that long cables will reduce the higher frequencies because the cable itself is a capacitor that will shorten these higher frequencies to ground. A cable is also an inductor and produced a magnetic field that will block these higher frequencies. There is energy loss in the cable. The amount of is a variable of the length and the geometry of the cable.

But this is also not a point at a length of 500 cm and this frequencies so you can do it.

More technical info:
Type of coax cable normally being used is the RG-58.
http://www.w4rp.com/ref/coax.html

The best choice for 50 Ohm and for minimum loss is the RF-9913 Low Loss Coax.
http://www.radio-ware.com/products/rf910.htm

For more details, please check the link:
http://www.spooky2.com/forums/viewtopic.php?f=23&t=5060