iPhone :How to get duration of video selected from library?

I am using UIImagePickerController to choose video file from library. And user can upload the video.

Also I am using videoMaximumDuration property while user wants to capture video and upload it.

I want to know that How can I get the duration of selected video file ? so that I can restrict the user to upload video that has duration more then 20 seconds.

I am able to get some basic info about selected video by this code :

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
    selectedVideoUrl = [info objectForKey:UIImagePickerControllerMediaURL];
    NSError *error;
    NSDictionary * properties = [[NSFileManager defaultManager] attributesOfItemAtPath:selectedVideoUrl.path error:&error];
    NSNumber * size = [properties objectForKey: NSFileSize];
    NSLog(@"Vide info :- %@",properties);
}

But there is nothing about duration of selected video.

Thanks...