Soon-to-be-published research on PTSD in veterans found that in nearly 40% of the subjects, the abnormally high activity of the amygdala returned towards normal levels after only one session of TM. Most of the remaining subjects showed a more gradual return to normalcy over a period of months.

As long as the subjects were regular in their practice, this change persisted throughout the 4 years of hte study.

So no, and yes: in some people, the benefits from TM practice can be immediate (from the very first 20 minute session), but regularity of practice is required for those benefits to persist.