Yes, most number transfer PINs have an expiration date. You must request a new PIN if it has expired. The validity period varies by carrier. For the big three carriers the PIN is valid for approximately 3-7 days. See below for carrier specific information.

AT&T: The number transfer PIN must be used within four days.

Verizon: The number transfer PIN must be used within seven days.

T-Mobile: The number transfer PIN must be used within seven days.