As such, I don't believe there's ever going to be a need for a 5e.
Technically they don't have to market a new "iteration" of D&D as 5E.
A new iteration of D&D (whether it will be called 5E or not) may very well be inevitable, assuming D&D is not taken off the market. The decision to pump out a new iteration of D&D, will most likely be driven by the accounting/finance people and upper management at WotC.
Going back in history, according to the "[ame="http://www.amazon.com/Wizards-Presents-Classes-Dungeons-Dragons/dp/0786948019"]Wizards Presents: Races and Classes[/ame]" book on page 8, they mentioned that work on 4E D&D started in mid-2005. So from start to the release date of 4E PHB1, it took around three years.
For 4E Essentials they had a one year "rushed" turnaround, where they started working on it back in September 2009. This is according to the July 2010 WotC D&D podcast, mentioned at around 00:00:55 in the podcast.
If the next iteration of D&D is just a clean up and/or refinement of 4E D&D, most likely it won't take as long as three years, but will most likely still take at least a year (by past precedents).
If they started working on a new iteration of D&D today, most likely it will take them two or three years from start to release date (ie. 2012 or 2013 respectively), depending on whether it is respectively just a "cleanup + refinement" or a completely new design.