When should I stop the medical heroics for my Mom?

I feel that we are at the end of the road and should now just keep her comfortable. Here's a little background - my Mom was formally diagnosed with Dementia a little over 4 years ago. 3.5 years ago she was diagnosed with Bladder Cancer. After a hospitilization due to the bladder cancer I made the decision to move my Mom in with us to better care for her. Due to her dementia she was unable to follow doctor's instructions, was messing up her medications and was also forgetting to turn off the stove. Moving forward to now - she no longer socializes with us or calls my 4 girls by their names (not sure she knows who is who anymore). Her walking gait has become different. She would sleep until noon if I let her. She no longer knows what day of the week it is or what season it is. When she does talk with us it is only things from when she grew up - she will be 80 this November. She has had too many bladder biopsies for me to count and she has had chemo treatments directly in the bladder and 2 rounds of BCG bladder treatments. The cancer came back - in June we completed 18 rounds of radiation treatments. She just underwent another bladder biopsy and again the cancer is back. She has high blood pressure, a leaky valve that has gotten worse since last year and diabetes. Her cardiologist has told me that he won't open her up again (she had open heart surgery in 2008). She is not a candidate for bladder removal or chemotherapy due to her health issues and she cannot have anymore radation.