  Entropy 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.
