theres no intereval to change it at, get it changed asap and flush it 3-4 times with couple hundred miles inbetween changes to flush all the crap out. its not cheap as the fluid (texmatic 7045e) is expensive and your gonna need 6 ltrs and a new filter every time you flush it, after its been flushed change it yearly. thats how i do mine and touch wood 94k towing the 27ft plasic palace often and no problems yet.