2003 Mazda 6, how often to replace transmission fluid?

I have a 2003 Mazda 6 and I was wondering how often I should change/replace the transmission fluid? I've heard so many people saying "don't ever change it", "change it every 30K miles", "change it every 50K"... I checked the manual and it only mentions "checking the level of the fluid". So ... I'm really confused! I want to keep my car in the best condition as I could and it seems like there are more cons than pros for having it replaced. All the experts out there, what's your opinion on this?

Many thanks!