Calling createSound with the info from getSoundInfo will open an FSB, after which you need to call getSubSound. Then you should be able to get the length of this sound.
(After createSound)
FMOD.Sound subSound;
dialogueSound.getSubSound(dialogueSoundInfo.subsoundIndex, out subSound);
uint length = 0;
subSound.getLength(out length, FMOD.TIMEUNIT.MS);