I'm going to say never.
Someone I know that is an engineer at GM powertrain group told me that is worst thing in the world to do. You get whatever trash the last guy had in his trans flushed into yours. The machine they use is not fool proof. It can run backwards and normally poorly maintained.
He suggested merely draining and refilling.
Sometimes there may be a filter to change.
So we brought up a total fluid change and devised a system much like a flush but with our own clean machine.
We take a 5 gallon bucket and a pal, take one line off the radiator (Trans line) And put a nipple with a hose into the radiator where it was taken from, Put both ends into a plastic bag and start engine for a second, The hose that pukes the fluid is the exit. With that determined you take that hose and put it into a suitable container, (I used a suitable sized box lined with two 55 gallon garbage bags) Now, ya take the other hose, that draws and you put it into the (Cleaned squeaky clean) 5 Gallon bucket. Have the remainder of the capacity of fluid standing by to toss into that bucket, this going to happen fast! Start the engine, dirty used fluid pours into the trash bag! Clean virgin fluid gets sucked up out of the Bucket! Keep that hose drawing clean fluid up to and a little over the capacity. When you ran say 13 quarts into a trans that takes 12, You have just completely changed the Trans Fluid totally, Lines, Pumps, pan, converter and all. Just like a flush machine without the metal particles from they last guy who was there because he had problems anyways.
Now you replace the lines to the radiator, start it up and check level, and for leaks.
Then take it for a ride and re check.
Now, Heat is your major cause of problem on a Trans and what trashes the fluid. We have a very up to date special trans fluid. I know it is more durable than 30K I'm not even going to jack with mine until at least 75K.
Whatever you want to do, use the Nissan Fluid as it is something so special no one even has it aftermarket yet. It has to be synthetic and that would make it very durable and resistant to heat damage.
Check on the filter, it may be easy to service when changing the fluid.
1st Time I changed mine on a Caddy was like 150K and everything was fine, fluid and filters. So personally, I think it's too soon to worry about it. maybe just check the level. MC