I have an 06 5at and I just did a flush at ~75k (really
hoping they did it at certification at ~50k). I'm borderline paranoid because of all the problems with the early 7gens, and the 7.5gens typically haven't gotten to the 130k-ish miles where the log thread is littered with failures.
With that said, let's recap:
A 1x drain/fill takes 3qt ATF and is definitely easier than an oil change. The fill bolt is only slightly tough to get to because it's so far down; a long extension solves this problem immediately (also why you need the long funnel). If you're paranoid, do this with every oil change. If you're moderately concerned, every 15k. Most dealerships will charge $100 for this, though it costs all of $24 in fluid (retail prices, you can definitely get the stuff for $6/qt).
A tranny *flush* as others have said, takes 3x drain/fill + another fill, for a total of 12qts. This is because the tranny really has 7qts in it, but only 3 come out on the drain. To mostly replace the fluid, you have to do the exchange 3 times - AND drive it between each time to completely mix the fluid. Alternatively, you can sit it on jack stands and run the car through all the gears each time (much quicker). At 55k, I would highly recommend you do the flush, but I don't doubt that the dealer would charge you about $500 for the service given that it's more time consuming than difficult. Total cost in fluids, though: $100 shipped. A good interval for a flush is every 30k (which is why I highly recommend it since you're nearly double) if you never do any other changes; maybe 45k if you drain/fill at 15k; paranoid people will still flush at 30k despite constant drain fills
As for the ATF filter: it's probably a good idea to change it at 55k, though you're probably going to have to do that one yourself, or fight with the dealer to convince them that it exists in the first place, much less agree on a reasonable price to do the change. Word of advice: look up the name of the service manager
and always to talk to them when possible. In my experience, the service advisors are typically trained to write you up for retail prices and not much else. Service managers can actually be reasoned with and are much more pleasant...