Even after 8 weeks according to this study. Sorry, it isn't a TM study but a "mindfulness meditation" one (think the study has been kicked around here already). However I posted the same would be true with a lot of different meditation practices. It's enduring because it rewires the nervous system and that will be permanent even if you stop after a while.
http://www.rawstory.com/rs/2012/11/12/just-8-weeks-of-meditation-can-cause-enduring-changes-in-the-brain/
