I am so sorry, Lisa.
Everyone always said it, and I never believed until I experienced it, but you will know when it's time. Brody had been happy and playful even with lymphoma for several months, but one day he woke up and wouldn't eat and wouldn't play and the light had gone out of his eyes. It was an instantaneous change, just overnight, and I knew. I put it off until the next day, hoping he would turn around, but he didn't.
Whatever decision you make will be the right one.