We present the novel task of predicting temporal features of continuations of user input, while that input is still ongoing. We show that the remaining duration of an ongoing word, as well as the duration of the next can be predicted reasonably well, and we put this information to use in a system that synchronously completes a user’s speech. While we focus on collaborative completions, the techniques presented here may also be useful for the alignment of back-channels and immediate turn-taking in an incremental SDS, or to synchronously monitor the user’s speech fluency for other reasons.