gah, yup... another time that I've confused PnP DnD and NWN.... in PnP, the duration is one round, and after digging up the instruction manual for NWN to verify Vyris' claim, I see that the duration is indeed three rounds in NWN.
P.S. thanks for the advice, Vyris, although I did already know that constructs and undead are immune to stunning attacks, for essentially the same reason they are immune to critical hits. I DM and play PnP DnD on a consistent basis and have for years, which has it's advantages here as well as it's disadvantages as demonstrated here.