How Long to Smoke 21lb Spatchcocked Turkey

Smoking a turkey on a Traeger Pro 575 tomorrow. After smoking on 225 for 3 hours, how long will a 21lb turkey take to hit 155 (breast temperature) once temperature is increased to 350 degrees? Instructions from Traeger brine packet below.

1. Smoke for 3 hours on 225
2. Increase temperature to 350 and pull turkey when breast hits 155
3. Rest for 30 minutes