You will know when it's time to let him go peacefully. I know it sounds cheesy but it's true. I had a dog with CHF and one day we just knew. Personally, I knew I was being selfish and I knew he wasn't enjoying his days. His little body was just giving out. His heart meds caused kidney failure so he was on fluids and was having accidents all of the time. He slept most of the time and couldn't do the things he loved. I'm sorry you're going through this and I wish you all the best.