I think the only diamond challenges are offered to diamonds who did not achieve their status and they are giving them an easy way to get back. As a gold, unless your diamond status just expired, I don't think they will offer one.
The last time I remember seeing one it was seven stays or fifteen nights in 90 days from the date you registered.
Status earned through a challenge is usually only good through that calendar year, not the following year like status earned the regular way.
Of course, if you call and ask for a challenge you may get somebody who offers you something totally different, or they may say sorry.