What's the duration of the stunned condition caused by the thunder power of a rod of thunder and lightning?
I would say for 1 round. It is caused by the noise. So think of when someone jumps out and scares you or a loud noise scares you. You become stunned for a few seconds then you reach out and slap them or yell at them or whatever.
I agree - I'd rule that it lasts 1 round, taking into consideration the effects of sound burst and greater shout, and the fact that it's pretty hard to make someone stunned for more than 1 round, on the whole (the consequences are usually fatal).
Also, in 1E (Unearthed Arcana) and 2E (DMG), the duration is essentially 1 round (no actions this round, and act last next round).