A completed appraisal (containing the guaranteed trade-in price), which is not yet linked to a new vehicle order via will disappear from the picklist in the Trade-in price proposed dashboard tab at midnight on the last day of that month.
However once the file has switched to this "Expired" status, you may still link new vehicle orders to that guaranteed trade-in price/appraisal file for up to 40 days after the expiration point.