It can vary, depending on just about everything you can think of! Which game it is, how many other uploads are in the queue ahead of yours, the type of upload (as an extreme example, neighborhoods and worlds will obviously take longer to review than wallpapers), etc... There's no set timeframe even for the same kinds of uploads though... when I was submitting TS2 lots for review, it took anywhere between less than 24 hours and just over a week for them to be reviewed.
Welcome to the Dark Side...
We lied about having cookies.
I assume you are talking about the challenge you just posted this morning. It can take a little while. But it will be gotten to as soon as it can. There are other older posts that need moderated first though.