Hey 6 new challenges go up every wednesday, and a spin a lift goes up every sunday. The challenges repeat every 2 weeks with new topics and you are eligible for reward points for the challenges for the whole month. Not really a deadline on the challenges as long as they are completed w/in the month. The last few challenges if they don't get a full week to run then they are carried over into the next month.
Check out the challenge tracker and you can get an idea of how they run and the points, etc. Our resident awesome guru Karen keeps up with all of that.