in the case of brake system flushing, your climate and usage habits make a huge difference. I grew up in the cold northern plains states, and cars there could go as much as a decade without the fluid absorbing a serious amount of moisture, but here in Tidewater, Virginia, the combination of humidity and heavy braking on overcrowded roads means change it OFTEN. I suspect that the factory recommendation assumes Autobahn and Alpine driving in what is also a rather damp climate. If you're in an Arizona retirement community, on the other hand . . . .