2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/125271Entropy creation rate is introduced for a system interacting with thermostats ({\it i.e.}, in the usual language, for a system subject to internal conservative forces interacting with ``external'' thermostats via conservative forces) and a fluctuation theorem for it is proved. As an application a time scale is introduced, to be interpreted as the time over which irreversibility becomes manifest in a process leading from an initial to a final stationary state of a mechanical system in a general nonequilibrium context. The time scale is evaluated in a few examples, including the classical Joule-Thompson process (gas expansion in a vacuum). The new version (n.2) contains several comments on references pointed out to me after posting the version n.1.6 pages 1 figureStatistical Mechanicstext