The Journal of Operative Dentistry has released a 2013 study which examined the shade changes in Vitapan Classical Shade Guide tabs. The study found that "the colour changes in the shade guide tabs were perceptible or noticeable to the human eye in eight out of 45 shade tabs after two years and in 13 out of 45 shade tabs after three years of treatment".
So what can your practice do to minimize inconsistent shade determination? The study suggests keeping one shade guide as a "control", and regularly examining for any changes. We examine and replace our's regularly in our shade-taking room for the purpose of consistency.