def _get_acq_time(start, default_value=-1):
    """Private helper function to extract the heuristic count time

    The SPEC-style plans inject a heuristic 'count_time' (which has
    different meanings in different contexts) as a top-level key in the
    RunStart document.

    Parameters
    ----------
    start : Document or dict
        The RunStart document emitted by the bluesky RunEngine or a dictionary
        that has compatible information
    default_value : int, optional
        The default acquisition time. Defaults to -1
    """
    time = start.get('count_time', default_value)
    if time is None:
        # 'None' is not legal in spec
        time = default_value
    return time