I'm sorry to hear :-(. I hope the meds are able to do their job well and make her comfortable. Cancer is a difficult thing, and it can be difficult to figure out when to make the decision for a canine companion. What I've learned is that they tend to tell you in the end when they are ready to stop fighting and desire a release. Until then I hope you are able to spend time with your beloved Duchess and go on walks, play with toys, and have fun as her health allows.
Also, I've also learned that opening one's home to another dog as soon as possible helps to ease the pain. If you feel its too soon to make a life long commitment, then I'm sure a rescue in your area would be happy to have you foster a dog.