total duration of open files

When I record audiobooks I bill the publisher based on the total finished length of the book. Currently I have to add up the length of all the chapter files by typing them manually into a spreadsheet to get the total. Is there a faster way to do this? For instance, by either:

a) seeing or calculating the total duration of all open files in Audition; or

b) copying the values in the duration column of the "Files" tab for easy pasting into a spreadsheet?

Any other ideas are also welcome.

Thanks,

Brian