I don't believe there are any judged /timed MTS site challenges going on right now. Those they usually post about in community news here.
The challenges that some individuals post can be found under the specific game and should list the rules for the challenge in the post. Here are Sims 4 challenges Link. . Here are Sims 3 challenges Link . Here are Sims 2 challenges Link . I think you just do these on your own by following the rules set forth in each of these challenges and post your results in the challenge thread.