Your POE letter will contain a section entitled "Permit validity" that will inform you of how long it is valid for. If the validity date expires, you need to submit a new application.
Validity lengths for International Experience Canada (IEC) participants only is usually 12 months. If you took a medical exam, your POE Letter will be valid until the expiry date of your medical exam or for 12 months (whichever of the dates is less).