08:34:48 PM Dec 20th 2012
December 20, 2012. Checking nethack.org shows no new release after v3.4.3 on December 8, 2003. That means nine years without a release. The previous longest time without a release was three years. Maybe there will be a new release on the ten-year anniversary of the last release, or maybe we're the victims of Neglectful Precursors.
11:23:03 PM Apr 26th 2016
There now has been, in fact, a new release. The developers emerged from their decade-long slumber and delivered unto us unworthy souls the glory of NetHack 3.6.0 this past December. The most notable changes are MASSIVE nerfs to Elbereth, significant buffs to the scroll of confuse monster, and monsters now have a random variance in their speed, so you can't rely on them having exactly X turns to your Y.