I don't think you can assign a standard dimension - within each school there is a wide variety of architecture that varied not only from maker to maker but also they cover a wide timeframe. If you look at the Kennedy group of rifles alone you see about a 40 year span and cheekpieces ranging from none, to fairly prominent. As a general observation on Appalachian (not just NC) rifles it was not uncommon in the very late period - i.e. second half of the 19th century - for cheekpieces to become almost non-existant as Bivins pointed out but this is not true in all cases - some of these latter 19th century guns still have some highly developed prominent cheekpieces and I think it primarily has more to do with specific maker preferences and skills rather than a general trend. Overall I think it is more important to approach it - as the original builders probably did - on what looks right and in proportion with the rest of the gun.
Perhaps Michael Briggs will see this and provide some more thought?
Best regards
Guy