:( I'm so sorry it's come to this. You and Denim put up a fantastic fight.
If he is laying there, breathing hard and unable to surface then it will probably be a matter of hours. However, if you can't bear watching him like that (and I don't blame you at all), there are some methods of euthanasia that you can use. The most frequently used one is clove oil. It is used to gently put the fish to sleep. The fastest and most humane methods are blunt force trauma followed by decapitation. :( Those are the hardest methods to use.
If you need any more help, let me know. I'm really sorry, qscoobs.