Usually I have my fluid and trans filter changed every 30,000 miles as this is much more thorough than flushing because some of the dirty trans fluid will be trapped in the system and the filter will still have debris trapped in it
dont flush unless they do it from the pump and remove the pan. The other is usless, it leaves a wore out filter in the transmission. I dont do the flushing at all, i get the pan cleaned, the filter changed and new fuild, yes this way leaves a little fluild in but it runs thru a brand new filter tp strain it plus most the fluild is new . what good is flushing when your old filter is still in there, that like changing the oil on your car and leaving the old filter on, they wear out and do nothing doing that..