One way is to download flaskmpeg (used for making divx movies).
Press file--> open dvd, (you put the .ifo file) and then it tells you the length of the movie and some other stuff.
Then you can close the program.
you can download flaskmpeg from:
www.divx-digest.com and
www.doom9.net if you want.
I bet there are a lot more easier ways to find out the length of your movie, but that's what came into my mind just now.
Hope I 've helped you.