transmission fluid on 2008 Mazda Mazda6

At 25,000 miles my Mazda dealership says my automatic transmission fluid is burnt/contaminated. Is this unusual? When should I change my transmission fluid?

Asked by for the 2008 Mazda Mazda6
To get the maximum life out of your transmission I would change the fluid and filter at least every 30,000 miles or what your owners manual states for maintenance