For how long would a very large tooth filling be sore?

Varies. I would contact your dentist if it is still sore after a few days. The bite on it may need to be adjusted. It is wise to keep in contact with your dentist so the best course of action can be taken.
Should not be long. Usually dentists do not place a very large filling since the damage from the large cavity weakened the tooth so much unless it's a temporary fill. A very large filling will not last long. Also a prolonged soreness that worsen daily is a sign of the tooth is infected already. Then you may need a root canal. But please have it checked and diagnosed properly.