...the average may be 72 hours. My personal experience has been anywhere from an hour to a week. My first review was one of those that took a little more than 24 hours. Lately they have taken from 24 to 36 hours.
There are so many factors involved, it's why we can only say 72 hours as a stated aim. In essence, it all comes down to how many reviews are in the queue ahead of you when you submit - if it's less than a couple of hundred, it will most likely be done within 24 hours, but more than that and it will take longer. Other factors include what time of the year it is (some months have more reviews coming in than others), what day of the week you submit the review on (fewer staff in the office on certain days) and indeed even what time of day (time zones and office hours and such). If you were to submit one right now, I would estimate (and it is only an estimate, please bear that in mind and don't take this as gospel for all reviews from this point forward) you'd be looking at closer to 48-72 hours.
Of course, the quickest way to get a review approved is to get it right first time. Make sure you read and follow the guidelines closely, as your review being rejected multiple times will slow down the process more than anything else will.