whitelighter,
Don't know when there will be more dates (HR can help you with that), but the rule with the system is that if you don't pick a date within 2 weeks of the invite email, your application is withdrawn. If it gets close to 2 weeks and there's still no dates you can do, just give HR a call and they can extend your application to let you wait.
My advice would be to wait a week (but keep checking in between), see if there's any more dates and if not, call HR.