Most manufacturers do not specify it. however the fluid deterioates and fine meatllic particles from wear, particularly in the pump circulate increasing wear.
I change my power steer oils. couple of options, pull feed hose of reservoir, drain, top up, run engine and turn steering lock to lock. do this several times and you will give it a fairly good flush, Or Pull the return hose off, run engine with this hose in a bottle while pouring new fluid into the reservoir, while turning lock to lock. 2 person job usually and a bit messy.
At the end of either, make sure the fluid level is correct and any air has escaped the system.
Regards
Greg