Never?
1) Users didn't ask for flat design
2) There's absolutely no evidence that flat design are better and there are evidence that they are worse
Yet I don't expect that UI will go back to 'classic UI', many of those who create UI care more about being perceived as modern (or more accurately not being perceived as outdated) than efficiency, so flat design will probably be replaced by another 'fashion driven design'..
Just like the current inexplicable vogue for illustrations of humanoid figures with bizarre body forms. Which designers seem to love and be very proud of using.
Yet I don't expect that UI will go back to 'classic UI', many of those who create UI care more about being perceived as modern (or more accurately not being perceived as outdated) than efficiency, so flat design will probably be replaced by another 'fashion driven design'..