Deciding when the right time is, is extremely personal and emotional. We really need to make sure we are doing it for the right reason and that reason, imo should be for the benefit of the pet, normally to ease the pet's suffering. Please keep in mind that dogs can still be 'happy' even if they have an 'accident' in the house or don't eat as well as they used to.
Because of this, I really like this (fairly objective) article on when the right time is. I hope it can help you. http://www.pets.ca/dogs/tips/euthana...e-pet-tip-228/