For Windows, see: http://answers.microsoft.com/en-us/windows/forum/windows_7-files/can-i-find-the-total-play-duration-of-all-music/b34a810b-2c77-4491-a912-2a01cb4f3c27?auth=1
You can select the audio files in a folder, then
1 look in the Details pane at the bottom of your Windows explorer window [switch this on in Organise, Layout, Details pane]
2 right-click, Properties, Details tab.
Both ways will show you the total length of the selected files.
If you want to include subfolders then you'll need to do a search on the top level folder of interest - search on type: mp3 or wma or whatever format you use and it will return all the corresponding files within that folder tree. Select them all in the search results window and then as I've written above.
If you have used a mixture of audio formats, you'll have to repeat the procedure for each type.