Considering XP lasted for 12-13 years, you can reasonably expect W7 to go for at least 10 years. Unless W9 is a complete turnaround to W8, W7 should be safe for awhile.
windows 7 will be the gamers and desktop users choice for as long as it takes, if windows 9 dont fix things efforts will be made to prolong the life of windows 7 until a better successor comes.