The 1 year extension is only used to serve as permit to work and travel "while your 1751 petition is being processed". If a denial decision is sent to you, I believe you have an option to appeal (I am not really sure how that works). If no appeal is made, removal proceedings will start and you will be asked to leave the country. How long does that take? I have no clue. Although, if you have no intensions of appealing after denial, it would be best to leave volunteering than be deported.
This is completely my understanding - experts can correct me if I am wrong.