Probably the most important principle in all of agile development reads like this…
Our highest priority is to satisfy the customer
through early and continuous delivery of valuable software.
I like to sum this up in one succinct statement, software that is sitting in QA is not providing value to anyone.
The most important thing an agile practitioner can do is identify valuble pieces of software and get them to production as fast as posible. Notice that this principle does not specify a timebox, or iteration length or any other such thing. The key is in the term continuously. As soon as you have something that could be providing value, get it out the door.