Add the folders containing the mp3s to the library and then it will tell you the playing time, or select the particular albums, genre or artists you want and it will show the total number of files and the playing time.
Don't think it's free now,but a very good program for doing anything you want with mp3's is dbPoweramp from Illustrate: http://www.dbpoweramp.com/
Just hover your mouse over an mp3 file and a box opens and gives all the info you need.
Stupid question? No such thing!
Virtual Dr. to the rescue!
Just ask. Bookmark your post for easy reference.