Specific post operative questions need to be addressed to your surgeon. Every surgeon has specific concerns related to the unique procedure they've done on you. They may feel you need longer time of elevation of the head based on what they did and a generic answer from a non-treating surgeon may be irrelevant. Sleeping in this position longer than you need to would not be a problem, but if you were to decrease the duration of abiding by this practice, you might increase the risk of bleeding, swelling and interfere with healing and prevent yourself from getting the best aesthetic outcome.
The amount of time to stay elevated after any head and neck surgery is dependent on the amount of bruising and swelling you may have. The main reason to stay elevated is to ensure rapid resolutin of your swelling and bruising.