There is a little software tool called anim-2-bvh that helps you to know the duration in seconds of an animation. Given that you have an animation file with a file extension equal to ANIM or ANIMATN, you can use AninToBVH.exe to convert this file into BVH format. This conversion tool will also create a text file with some information needed to upload the animation into Second Life with the same parameters the original file, like priority and hand position. The text file also has some usefull information like animation duration in seconds.
So you see, if you have animation files saved in animatn format you can easily get the animation duration ;-)