First, you can do a quick check on your tranny dipstick and put some of that oil on a white paper towel. See if the fluid is clear and red, or a dark red. Then smell the fluid to see if it has any burnt smell.

If the fluid is a dark red, or burnt smell, you should change your fluid now. Also most will recommend you only do a drain and fill, and not a power flush by some shop. Power flush may draw up some metal shavings from the bottom of the pan and do damage to your transmission. With the drain and fill, you will hear people tell you that not all the fluid is removed because there are still fluid trapped in the torque converter...this is true, but if you do your own work, you can do a second drain and fill after couple of days of driving. This will circulate the fluid fully and get the remaining old fluid mixed with the newer fluid.

As for mileage when to change. I think you should get it done between 40-50k. At 60k you should definitely get it done.