Overall capacitance of the sytem, including the nodes. Each device has it's own capacitance, not only the cable.
As Nick already stated it is imortant to dimension the pull-up resistors correctly. the higher the bus speed (Clock Frequency) the more critical these things get.
here's a very nice articlke on the importance of the Pull-up resistors:
Also, you can certainly experiment a lot, however, if you encounter problems, without an oscilloscope you won't really know what your problem is.
Search the forum for "I2C lenght" or "I2C distance". There's some good stuff posted.