Do Contact Hours have an Expiry Date?

No. PMI does not mandate any expiry timeline for Contact Hours (CHs) earned to qualify for the PMP Exam, as long as you retain the proof (certificate). For example, if you earned the Contact Hours in 2005, there's nothing stopping you from taking the exam in 2010.

I quote a message that I received from PMI on this matter:

"There is no deadline or expiry date for the PM education requirement, though the PM education must have been acquired before the submission of the PMP application."