The Arachnid Quarter of Curse of Naxxramas has been available free of charge since the game mode's release. However, we knew that was only a limited time offer. Now that all four quarters of Naxxramas have opened, Blizzard has announced the offer will end September 30 at 11:59 p.m.
Beginning October 1, if you have yet to access the Arachnid Quarter, you will need to spend real money or in-game gold to gain access. That gives you a full month before the offer concludes. Even if you aren't a current Hearthstone player but think you might pick it up one day, we recommend popping into the game before September 30 just long enough to open up the Arachnid Quarter. It only takes a few moments and it's better than paying for it down the road.