By default an flv movie doesn't contain the duration metadata. Using the flvtool2 program it is injected like this into the movie file.

cat mymovie.flv | flvtool2 -U stdin mymovie.flv

A very sophisticated algorithm that will display the length of, for example, a video as 12:01:30.

function duration($seconds_count)
    {
        $delimiter  = ':';
        $seconds = $seconds_count % 60;
        $minutes = floor($seconds_count/60);
        $hours   = floor($seconds_count/3600);

        $seconds = str_pad($seconds, 2, "0", STR_PAD_LEFT);
        $minutes = str_pad($minutes, 2, "0", STR_PAD_LEFT).$delimiter;

        if($hours > 0)
        {
            $hours = str_pad($hours, 2, "0", STR_PAD_LEFT).$delimiter;
        }
        else
        {
            $hours = '';
        }

        return "$hours$minutes$seconds";
    }