So an Eldritch Knight type casts Heightened(+1) Dazing Shadow Weapon and attacks with it.
It seems to me that the first attack's will-save-to-disbelieve would also apply to the daze, while subsequent attacks (whether for full damage or 1 damage) would use subsequent will saves; and that with an effective "real" spell level of 2 (heightened), the daze would last 2 rounds. But I'm not sure others would necessarily agree...?
I believe I've read somewhere (perhaps someone's opinion on the forums) that dazing only works the first time/round of damage but I can't find anything but I can't find it so as far as I know that's correct by RAW
You can modify a spell to daze a creature damaged by the spell. When a creature takes damage from this spell, they become dazed for a number of rounds equal to the original level of the spell. If the spell allows a saving throw, a successful save negates the daze effect. If the spell does not allow a save, the target can make a Will save to negate the daze effect. If the spell effect also causes the creature to become dazed, the duration of this metamagic effect is added to the duration of the spell.
It doesn't matter if the base spell has a save or not. The saving throw occurs whenever the target deals damage from the cast spell, but there are key notes between the saving throws.
If a Spell doesn't offer a Saving Throw, affected creatures get a Will Saving Throw to negate the Daze.
If a Spell does offer a Saving Throw, affected creatures treat the saving throw of the spell (which can be Fortitude or Reflex) as the saving throw required for the Daze.