Here is a bit of info from the tunecore website about how quick you can get listed on the big MP3 sites:
On average it takes us about 30 minutes to deliver a release.
Currently, the average time for music to appear 'live' in stores is
* iTunes: approx. 72 hours
* AmazonMP3: approx. 4-5 days
* Myspace Music: approx. 7-10 days
You could get a lot more exposure for sure
