This is what Marco Polo told me

• Bookable Upgrade entitlements are issued upon achieving 1600 club points
• Bookable Upgrade entitlements are each valid for 12 months.
• Bookable upgrade entitlements must be claimed within 12 months after it has been issued. The departure date of the flight for bookable upgrade is not required to be within 12 months from the date of issuance, however, this date must be 15* to 360 days after the claim date.

* not stated explicitly, but given this period it sounds like one must book the upgrade at least 2 weeks in advance.