Average time for approval of iPhone application on Appstore

Rather than giving an answer (which will fluctuate greatly at times), there is a page where Apple publishes the percentage of both new and updated apps that are approved within 7 days. They update this on a semi-regular basis and can give you a general feel for how long it will take. I have seen this as high as the mid-to-high 90s and as low as the 80s. It all depends on a number of factors, like the complexity of your application, how many others are in the queue at the time, is it close to a new release of iOS, and how grumpy SJ is feeling (just kidding about that one).

UPDATE:

Another great resource is this site from ShinyDevelopment. They crowd source their data from developers and can give a better idea on how long things are taking--something especially useful when Apple doesn't update their site regularly.