I called today to request a challenge, but was told I wasn't eligible. I assume this is because I completed a Platinum challenge in July of 2011, but just want to be sure there is a strict waiting period between challenges? They CS agent I spoke to said he thinks it's 5 years. I couldn't find anything in the Wiki except:
You are not allowed to use a Challenge to re-attain status earned through a Challenge within the same year.
I have RT J seats booked SFO<>SYD, so I was really hoping to double my miles for that. Of course, I understand you eventually have to earn status the hard way.
Edit: Oh yeah, I did end up flying enough EQP in 2011 to actually earn Platinum (~70k points), and flew enough in 2012 to actually qualify for Gold, as well. Not sure if those should have any bearing on when you can repeat a challenge.