I do this at least once a set but often more times in between as well to keep things fresh and in fitting with the meta game. These changes are primarily due to the release of Dark Ascension but a few other significant cards have also been added. I have paid little attention to colour and number of cards going in or out but there are some cards which are direct replacements as I know the cube cannot justify two slots for such effects. As my culls only go as far as the B cube it only effects drafting and is not at all permanent so I am pretty ruthless in culling and happy to try out fairly obviously weak cards.
Heartbeat of Spring
Hero of Oxid Ridge
Smash to Smithereens
Stroke of Genius
Brave the Elements
Edric, Spymaster of Trest
Sorin, Lord of Innistrad
Huntsmaster of the Fells / Ravager of the Fells
Master of the Wild Hunt
Crushing Vines (replacing Krosan Grip)
Torch Fiend (replacing Smash to Smithereens)
Faith's Shield (replacing Brave the Elements)
Thalia, Guardian of Threben
There are more cards I rated for the B cube which I am not yet testing as I don't have all the cards yet and am happy enough with the large quantity of other, far better stuff to be playing with for now. I suspect about half of the new additions to be cut in the next big cull and I wouldn't be surprised to see the return of a few cards I have just now cut such as Pestermite. I'll do an updated cube list at some point and try to work out how to get those flashy link popup images things working for it...