Not 10 years, remember Michael is always groundbreaking, new and innovative with his music. With every album, he manages to twist contemporary genres which are popular in that current music market into his own sound, which is much of Michael's appeal. If he took 10 years to make an album he would have to change the sound constantly throughout its writing and recording. So no, 2-4 years is a good time for an artist such as Michael to write, record, change, manipulate. re-write and re-record etc..
Also remember, that in essence, the promotion has already begun for his next project. Thriller 25, King of Pop and upcoming releases of collaborations with current artists are all ways in which Michael is getting back into popular culture in preperation for the release of his upcoming album.
We have only been waiting since around the World Music Awards in November 2006, since around that time he announced personally that he was working on new material and it was in the early stages. You cannot count the one or two year break after Invincible, where he probably just reaped the benefits of the album.. And of course you cannot include 2003-2005 because of the trail.. Since he was aquitted up until the point where he announced he was working on new material was just over a year, which is INCREDIBLE considering what he went through. By all means he could have become a complete recluse and not written or recoreded or done anything again INCLUDING T25, but he has decided to burst back onto the scene merely months after a traumatic experience for him. Give him a break!