I believe that Mountain Lion was at 10% after five weeks. OS uptake on Apple products is very fast. iOS 6 was much, much faster.
That's because a Mountain Lion upgrade is $19.99, while Microsoft charges $69.99 for a Windows 8 Pro upgrade. Apple also has an easy mechanism through the app store to upgrade their OS, while Microsoft is not intuitive to most laymen end-users.
Hopefully, future versions of Windows go through the Windows app store, and make it easier for PCs to keep pace.