Is it time to put my dog with cancer down?

I have a 6.5 year old beautiful 110 lb German Shepard. She was diagnosed with osteosarcoma in Jan 08. We immediately had her rear leg amputated. After much research we decided to do no chemotherapy. We were told her life expectancy to be 4-6 months. Now we are approaching 11 months and she is declining rapidly. She has had infections which have been treated, had been expelling blood (about 2 months ago), and has urinary incontinence, based on the fact that it is difficult for her to go out. She must rest after walking 10 feet. Her breathing then becomes labored. She is very weak and can no longer easily get her backside up to walk without us slinging her to help her. Last night she had fecal incontinence, because she didn't wake us to go out. I know she doesn't feel well, but she is still eating and barking. We have her under vet care and they are very supportitive. What do we do?