The rule of thumb is 1/focal length. Some say that you should take the crop factor into account.
So for your Canon camera, you would want a shutter speed that is at least 1/FL(1.6).
So when shooting at the long end of your 70-210, you would want a shutter speed faster than 1/336....and that might be pretty tough with a max aperture of F5.6.
This is only a rule of thumb though. Most people can easily hand hold at faster shutter speeds, with proper technique. For example; a good stance, good posture, holding the camera properly & good breathing technique.
When I'm shooting a landscape, I'll usually use a tripod anyway. This allows me to practically eliminate any blur from camera shake and also allows me to use a more optimum aperture...either to gain some DOF or just to use an aperture that will result in better image quality. I'll also use a remote shutter release (or the timer) and activate the mirror lock up function.