Here's a nice straightforward (read: brief) one for a change. Thanks!
According to the Las Vegas Convention & Visitors Authority's Annual Report for 2007, the average Las Vegas visitor stayed for a period of 4.5 days and 3.5 nights, so you're definitely ahead of the norm.
Those surveyed had visited Las Vegas an average of 1.8 times in the preceding year, spent $108.87 per night on lodging, $254.49 for the trip on food and drink, $114.50 on shopping, $47.57 on shows, $8.31 on sightseeing, and had a gambling budget of $555.64 for their visit. This compares to the 2006 figures of 4.6 days average stay and $651.94 budgeted for gambling. It will be interesting to see figures for 2008 when they emerge.