(Updated answer choice for High Sierra. See the old answer if you are still on Yosemite)
I'm running OS X Yosemite, and I want to disable the notifications from Time Machine. I rarely do a hard-drive backup because all of my essential files are in the cloud. I don't need to run timemachine as often as it suggests/warns/annoys me about.
How do I disable the notifications? Time Machine is not in the Notifications preferences, and I don't want to stop using it entirely.