There is indeed currently no way to do that in the quiz module.
One thing that could easily be done is to allow you to give the quiz a timelimit of one week. That would effectively set the deadline to be one week after they first view the quiz. Maybe that is good enough for your purposes? This would only require a small change to the quiz setup page which currently, for no particular reason, limits the timelimit to be no longer than the session timeout. I will change that for Moodle 1.6.
Quiz
time limits for students entering at different times
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