how often to change your transmission fluid

On a "Cobalt" you don't need to change it for the life of the car unless you ran into transmission problems because it's a sealed "tranny" you won't be able to check the fluid. I've owned my cobalt since I bought it in 2008 and I bring it to the race track, no issues with "tranny" fluid or anything like that yet so I'm assuming you don't race so you shouldn't run into any issues any time soon. Good Luck.

I'd say in the average range of 60,000 miles, but it depends on the climate, location and driving style also, therefor the fluid condition and level has to be checked periodically. If the fluid lost its color and has a bad, burnt odor at 30K miles, than better to replace it. Keep it in mind that a regular drain and fill method won't replace all of the fluid, quite often more than half of it stays in the torque converter, cooler and cooler lines, so some new fluid be mixed with more old one. The best way to replace all the fluid is you ask for a complete transmission flush, which is done by a flush machine. Make sure the shop will use the factory approved ATF! Zee