Well, you could use pen and paper and calculate yourself using the Speed and BPM values to your aid.

First, a row in speed 6 is equal to 1/16 note which 1/4 the length of a beat. Then if we have the BPM setting to 120, it will mean that 480 rows will pass in a minute. To get it as seconds, you will have to divide with 60. 480 rows/60 seconds = 8 rows/1 second

If the speed is different than 6, you can easily calculate the new BPM with this formula:

BPM*6/Current Speed

Count the amount of rows in this BPM and take in consideration of speed changes, pattern repeats and pattern break.

If there's a change of speed, just simply put the amount of rows you've calculated aside and start calculating with the new speed.

Do this until you're at the end of the song and add all your numbers together and you have the time in second. Then you can of course make them in hours:minutes:seconds with some quick calculation.

You could make a program which does this, too. It should be as simple as that.