7 years is a long time, but I think because of how intergrated it is in society as of now, that it would be even longer then this.
After all it's what people use at work, school, and home. The only reason for someone to even attempt to change is because they become disatisfied with it, and even then those are the ones that know the smallest amount about their PC's.
I don't believe Linux has a chance at taking the desktop market until it becomes totally automatic like Windows. Some of you may say this is already the case, but it is not. As of now most things are installed with source code. Until installation of all programs becomes as easy as Windows, then I don't think Windows is going to simply die away.
Just my opinion.