The time should decrease. Let's not assume the barracks is working as intended though. I've seen a few bugs with the barracks though but I've never sent in a ticket because I'm lazy.
Sometimes your troops in queue will disasppear out of it and you won't get resources back, other times the time won't change at all, and yet still other times you'll have the troops instant finish. Those instances are very few and far between, but you can imagine the rage and elation you feel with each occurrence. xP
I hope this helps in some fashion. I kinda rambled a bit...