Use the Text Tracking parameter controls in the Behaviors Inspector or HUD to control the rate and end offset of the tracking animation:
Rate: A slider to control the rate of the change in tracking values. The higher the rate, the greater the tracking values.
End Offset: A slider to offset the end of the effect of the Tracking parameter. By default, text characters track to the end of the duration of text. For example, to stop tracking animation 60 frames from the end of the text object (or project), drag the End Offset slider to 60 or enter 60 in the value slider.
Note: Use the End Offset parameter to stop the effect of the tracking behavior before the end of the text object (or project) rather than changing the duration of the Tracking behavior in the mini-Timeline or Timeline. If you trim the end of the behavior, the text snaps back to its default tracking value at the end of the behavior.