It gives Planks every 4h - increased by +1 from enchanting and another +1 if you allow a neighbor to harvest it for you. You can also get planks when you harvest a neighbors Lumber Mill or Large Lumber Mill
You must reach a certain Magic Level in order to upgrade
September 4, 2018 Game Update • Lumber Mill Changes:
Lumber Mill Production timer reduced to 4 hours (from 8h)
Lumber Mill no longer gives Poles. They were completely removed from the game.