You need to examine the touchPhase of each touch on the screen: http://docs.unity3d.com/ScriptReference/TouchPhase.html
In the frame in which the touch is first registered, this will be "Began". In every subsequent frame the touch is held, it will either be "Stationary " or "Moved", until it is ended or cancelled. So you can consider those two states as equivalent to a mouse or key button being held.
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