Jeni, I'm so sorry to hear that your dog has cancer. I am not familiar with the type of cancer it has, but we just put our 12 1/2 year old Lab down the end of October. We found she had a tumor in her left front leg and after spending a horrible night with her being restless, we made the decision to put her down the next day. The leg could have broken and shattered for no reason and we didn't want that to happen. It seemed to happen all to fast, but she was not happy and wasn't moving. It is a terribly hard decision to have to make, but we feel we did the best for Kelly...not us.
This is the second Lab we had to put down due to cancer. The first one, we found a lump in her chest area and had her X rayed and got the bad news. And found out the cancer was in her lungs, too. We brought Sammi home and had a good night and day with her and took her into the Vet's the next day for the last time. I know all of our pets are well and happy now and I still miss every one of them, and someday we'll see them at the "Rainbow Bridge". And yes, we have another Lab...Dakota.
Listen to what your Vet tells you and then watch your beloved puppy and see what he/she tell you...sometimes you just know when it is the right time. Hopefully, you will be able to keep him/her around for awhile longer.