In Agile, you are supposed to ship product on a regular cadence. All features are added to a product backlog, divided into small projects that fit within a sprint, and then prioritized. The backlog is just a way to keep track of all of the ideas people have, requests from customers, major bugs and other things.
The first patent for Pristiq is set to expire in February 2022. This is the earliest predictable date that a generic version of the drug could become available. However, other circumstances could come up to extend or shorten the exclusivity period.
As previously noted, Levitra's patent is set to expire in 2018, and Cialis has dual patents, one expiring in November 2017 and the other due to expire in 2020. Since Viagra's patent expiration is no longer an issue, the big question is how the patent extension will affect the marketplace.