I used to go into the challenges section and specifically look for monthly ones where there is a sign-up by deadline and once that month rolls around, you post your LO sometime before X date (usually at the end of the month). But then I automatically kept coming to this general section and I forgot to check challenges. I haven't done one in ages...more than a year! But it was a good way to get my creative juices flowing!
ETA: There might also be challenges in any local scrapping stores, or scrap nights you might attend. I go to one once a month and we hang out LOs that were completed sometime in the month before. I haven't hung anything yet because my current album is all double page LOs. And I went to a retreat in May that is twice a year. The lady who organizes it does a challenge in between May-Oct. to get us to keep scrapping between that time frame. We bring our challenge pages in and get a prize just for participating!