Final answer:
The concept enabling listeners to predict when a speaker might finish their turn in a conversation is projectability, based on linguistic and contextual cues such as syntax and intonation.
Step-by-step explanation:
The concept that allows a listener to predict the completion of a speaker's Turn Construction Unit (TCU) is projectability. Projectability is a feature of conversational turn-taking that allows participants in a conversation to anticipate when a current speaker's turn may be coming to an end. This anticipation is based on various linguistic and contextual cues such as syntax, intonation, and pause, enabling listeners to prepare for their own turn at speaking without overly interrupting the current speaker. For example, a rise in intonation at the end of a sentence can signal a question, prompting the listener to respond, whereas a falling intonation might signal the end of a statement and thus the potential end of the speaker's turn.