Time interval for TC/diff oil change?

[KWAWD]

I'm not using my cars very much and only putting k's on them very slowly.
Does anyone know what the change intervals should be for the transfer case and rear diff oils in terms of time? The book says 45k's, but at this rate that may take another few years. TIA

[an observer]

45k equates to three years normal use.

Outside that.. unsure.