Once weakened, always weakened I believe. I don't know if there's a timer for the effect. It doesn't matter though as with high enough weaken chance % and lots of hits, weaken will happen often.
Yes weaken does have chance % so 90% weaken means 90% of the time, enemies will be weakened .
Once you have 90% or more Weaken, you can deal more DMG than with Ignore Resist since negative resists could be why . Effective mythic is good with this for greatly resists enemies . Ascendant Set can reduce resists by 62.5% through arcane attacks (arcane debuff counts since wrath talent technically is an arcane attack ) and combined with 90% or 180%+ Weaken , it's very powerful.
You could effectively have doubled the potential damage from high enough weaken but ignore resist just makes sure you deal consistently the normal damage without it reduced by enemies.
Weaken probably counts as an arcane attack also when equipped with Arcanist so it could be great with Ascendant for further DMG.
Lastly, weaken isn't an elemental attack. Rather it's a debuff which is a negative effect to enemies but positive to the attacking player but it does reduce all elemental resists from the enemies by %. If with Arcanist, it can become an way to be an arcane debuff and could count as arcane attack to go extremely well with Ascendant.