You won't need NAudio for this one, but Windows Media.NET ( http://windowsmedianet.sourceforge.net/ )
You will open MediaReader, and retrieve so called 'samples' that are compressed audio packages. Each of them should have its duration. Traverse the whole file, total the duration of each package, and here is your solution.
I'll get some code if I find time for it.
More info:
Reader: http://msdn.microsoft.com/en-us/library/dd757425(v=vs.85).aspx
Callback for uncompressed samples: http://msdn.microsoft.com/en-us/library/dd743503(v=vs.85).aspx
I hope that you'll be able to put it all together!
One problem though, and that is one that drives my friend nuts - regardless of how much actual PCM is compressed in WMA file, last compressed sample that you get will always be 'padded' to full frame length, so if you want actual length that was given before compression, you can forget it.