"Replace transmission fluid" is Code 3 of the Maintenance Minder with the note
"*4: Driving in mountainous areas at very low vehicle speeds results in higher transmission temperatures.
This requires transmission fluid changes more frequently than recommended by the Maintenance
Minder. If you regularly drive your vehicle under these conditions, have the transmission fluid changed
every 25,000 miles (40,000 km)."
"Replace spark plugs" is code 4 of the Maintenance Minder.
So, in short, whenever Maintenance Minder shows codes 3 or 4.
Seriously, it's all in the OM. (That stuff is on pg 481.) They don't go to all the trouble to develop a 580 page book just for their own amusement. It's on the CD and the web. You can get a printed copy for free if you want one.
But if you are wondering if these things get changed every 3 months or 3,000 miles, the answer is no. But you can probably find a dealer or a quick lube place more than willing to tell you they should.