For example someone could be great at Hibernate but not really have some a lot of work with pure JPA. Just because they are not familiar with the JPA does not mean that they have not mastered ORM concepts and would be able to do JPA development as good as the rest of us in a week or so.
Which brings me to my next point. I would throw time completely out. With the situation I just framed a senior level developer is going to do what all of us do. Fire up the browser look at the JPA api and implement it. This is what makes a good developer. Someone who can gather code together and create a solution.
On almost any given project there are going to be new API's that developers will have to familiarize themselves with. The ones that can are far more valuable developers, because they disseminate that information back to the rest of the team.
Your test does not account for that and by making it time based you are disadvantaging the very ones that posses that valuable skill. If the answer is right, it's right.
As well you should consider offering the service to recruiter and hiring line managers, not as a competitive test, but rather as a filtering tool. Competitions are regarded in academia, but in the corporate world people just want to know if you can do the job.