import math


def describe_func_duration(n_vols: int, tr) -> str:
    """Generate description of functional run length from repetition time and number of volumes."""
    run_secs = math.ceil(n_vols * tr)
    mins, secs = divmod(run_secs, 60)
    return "{0}:{1:02.0f}".format(int(mins), int(secs))