For me personally, the time to put my cat down was when I realized that keeping him alive was solely based on my selfish wishes to keep him a while longer. The cat had cancer, and I knew once he decided to start hiding under my bed...a place he had never gone before, that it was probably time to keep him from any further discomfort.
Thats why I wait for them to let me know.You watch them and are around them all the time, you know when they are telling you they need it to be over. Until I get that from them, I help them.