On Windows, just select all the clips you want to find the total duration for and the length will be detailed in the bottom portion of the file browser in the information pane. We will happily support you through this process if you need any further assistance.
On a Mac, this can be done in iTunes. Just open up iTunes, click the ‘+’ icon to create a new playlist and then simply drag and drop all your clips in. The estimated duration will appear just under the name of the playlist near the top of the application. You can delete these files from iTunes afterwards (as long as the files are still backed up in their original place!).