Check with your dealership to see what is recommended for the vehicle. There is more than one thought on transmission filter and fluid flushing. Many say do it every 50,000 miles the other thinking on transmissions is if it is over the 50,000 by a bunch just leave it the way it is. Many times after changing it out the transmission will burn out soon after. I have seen that happen.

You don't have a helpful answer because you are not telling us what sort of car or truck it is for. Make, model, and year are everything for this. Most Toyota vehicles in the last two decades or so are never expected to get new fluid, and Toyota describes their WS fluid used since 2004 as "lifetime" fluid. On the other hand, Hondas, especially those from 1998-2004, should have fluid drain and fill (no filter to change) at most every 30K miles.

Every transmission has different service requirements. Since yours must be a secret, we can't help you.

Depends on driving conditions and vehicle manufactures recommendations. Usually not very often.

It isn't a "one size fits all" kind of thing. And not knowing what vehicle, mileage and driving conditions, I can only say start with whatever the "severe" column of the maintenance schedule states. More info about the car and the situation will likely change my opinion.

not surprised since I don't know make and model since some transmissions are drain and fill only

funny thing is some makers do not recommend service! look into it. if any of them wanted you servicing them, theyd include a drainplug and spin on filter.

About 65-70,000 miles will work.

I do mine every five years.

look in your owners manual.