If you are gentle on the car, I would say 50K miles would be ok to change the PTU fluid, otherwise 25-30K miles. If you engage the AWD rarely, RDU fluid can go longer, but I'd definitely change that out by 100K miles. Engage AWD often, I'd halve that to a 50K interval for the RDU fluid.
Drain the old fluid, flush a couple of times with parts cleaner/degreaser/motor oil/followed by a change of PTU fluid/RDU fluid, and when that drains out looking clear/good, put in the final PTU/RDU fluid as appropriate.
In short, the shortened intervals are particularly important the more tuned/aggressively driven your car is.