Betta Fish: When? When do I say goodbye?

When is enough? When do I have to say goodbye?

His fins are not as flowy as they once were because I can tell his health is starting to take a hit from the tumor. He's slowing down a bit. Still active but very clearly gets tired and will go to his spot behind the wood and lay down on the floor of the tank. I'm so torn up over how fast this tumor has spread and how much it's grown because I know it's going to spell the end for my boy. I'm not prepared for this but I knew my time would be short when I got him.