I've heard it said that "you'll know when the time is right" but in reality it's never that easy & we question & torture ourselves with the decision. I've been through this recently with an injured horse who just wasn't improving, & was in fact was getting worse. Be guided by your vet. A good question to ask - although vets probably hate it when we do - is "What would you do if he/she was yours?". When there are more bad days than good, i'd say that's a good indicator that it may be time. No matter what you decide, always remember it's better a month too early than a minute too late xx