Christian,
The answer is quite simple: Contact hour *never* expire.
This means that your 35 contact hour certificate is always valid. No matter when you take your PMP exam.
However... you have studied training materials preparing you for the current PMP exam. I recommend that you take your exam before July 31st 2013, because the difference between the current exam (based on PMBOK 4) and the new exam (based on PMBOK 5) will be significant.