I did see the related question
How do you know when an interface is complete - Though the answers in this question are very interesting,they are more from a product management perspective of what to keep and what to ignore with regards to a finalized product.
But can we really determine when a product is finally usable ? Is there a defined parameter like "90 % of the users found this usable" or "All the associated heuristics which are applicable to this application are satisfied" or "All the potential use case scenarios were accounted for with the user using the optimal path as opposed to using the alternative route"
Answer
This is in fact your decision.
You must define the usability goals for the project in the usability specification. They are usually formulated just like you suggest: "n% of the users accomplished the task in less that n minutes". A couple of "standard measurement" have evolved during the last decades. Measures like time-on-task, success-rate, error-rate, efficiency and preference are pretty common. Take a look at "Measuring the User Experience" by Tullis and Albert. They cover the most essential issues in a very comprehensive and straightforward way.
The usability goals are set by looking at existing systems, competing systems or prototypes. In some circumstances you can set a goal that "sounds reasonable" (i.e. by following your gut feeling), but you should usually have some experience with similar systems and some basic performance- and preference measures as a basis for your new usability goals. Even if the goal is pretty obvious, it should still be explicitly stated in the usability specification.
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