Maximum length between ION8600 and IO Expander is 45ft(13.7m) (under a controlled test environment).
IO Expander cables can be extended however it will require a specific cable configuration. For example if you want to extend the connection to a maximum of 16ft you will need to have a cable configuration like the following:

[IO Expander] [A] [B] [A] [ION8600(in breakout panel for example)]

Where:

[A] CBL-8XX0-BOP-IOBOX : is a 5ft male-male cable to make the IO Expander-ION8600 breakout panel connection
[B] CBL-8X00IOE5FT : is a 6ft male-female cable used to connect an ION Expander to a standard ION8600 with a breakout cable(which is the same as the male connector)

Addendum:

Refer to the most recent version of the Product Part Number Guide to verify the part numbers are up to date.
No official specification exists as this distance was tested under a specific application request. Longer connections may work however nothing beyond 45 has been tested.