Well to be honest - nnever?
I mean, I organized one a long time ago, but very few people were interested (and, fairly, since there weren't even any good rewards) and the group has very little regular activity, so I don't organize anything.
I guess that if I started a new content nowadys, I could actually offer some Points as rewards.
I dunno, I could make a post asking how many people would be interested. If I get enough of a response, I could make another contest