How often do I need to change the transmission oil?

CVT? I would do it every 50,000 miles or less which means you should do it even if the dealer says it is "lifetime".

FLUID AND FILTER CHANGED at 49834miles. Oil changed every 3,000 miles with Mobil one

I'm not so sure. I'd been changing CVT fluid on Gen 1 for years, but seeing only clean fluid drain out. With Gen 2 I'm following SOA's advice and leaving it alone. Gary, changing the motor oil at 3k mi is unnecessary unless that's all city trips over 6mos or more. I recommend changing motor oil twice each year, using 5w30 synth. I use M1 because it's cheap when on sale at COSTCO. Use a bigger filter, too, like a 4459, as it works better and fits nicely.

That is my dealers recommendation and I trust the service manager. Of course this is west coast, not east coast and we have really hot summers.

Even the 2nd Gen CVT? The fact is that we just don't know if changing it is helpful, as it's still a young'un. Failures of Gen 1 CVT seem to be related to bad inner bearings and TCs, not grimy fluid. This may be a case of individual managers squeezing profit or being overly careful. Maybe depends upon the price charged as a hint, as draining and refilling the CVT takes only a few minutes with a pump, so should be under $75-100. I'll be watching 2015+'s to see if any fail.

I got a 2015 with 200k on her. Never changed CVT oil and am contemplating if I should or not...

Interesting question. I might drain and refill just to LOOK at it to see if it's picked up dirt from friction. But I wouldn't fret over it.... Let us know how it looks.