100% condition duration
Anything over 100% is generally wasted.
The critical part of the wiki that Ayrilana linked to is:
Condition Duration, like Boon Duration, has a ceiling of 100%.
So… yes, anything over one hundred percent duration is wasted, because of a game mechanics decision. There is nothing other than the naturally diminishing returns of stacking larger and larger buffs that otherwise would make it wasted.
Yes/no, in general yes 100% is the max duration extension. However that’s for general condition duration. Lingering Curse and Chemical Rounds however let you go beyond for the scepter and pistol skills respectively. Not sure if there are any more traits like that off the top of my head.