How often are new iPod touch versions released? I am wondering whether Apple releases a new iPod touch version once or twice a year (like the iPhone S which comes after a few months from the original release).

Thanks.
A nice resource for questions like this is the Macrumors Buyer's Guide, which lists average time between product updates and a recommendation of whether to buy or wait. As the iPod touch is concerned, this product is listed with a last release date of Oct 9, 2012, and an average update cycle of every 342 days (approximately one year).