Mishra's Groundbreaker

• WWizards of the Coast2008-08-01
  Official Ruling
  A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.
• WWizards of the Coast2008-10-01
  Official Ruling
  The ability’s effect has no duration. The affected land will remain an artifact, creature, and land until the game ends, it leaves the battlefield, or some other effect changes its types.

Rulings via Scryfall