Based on the time you posted, Iím going to guess you have had your veterinary visit or itís really soon. You know there is really is no quick and easy answer. Unfortunately this is one of the hardest decisions we have to make when we care for these little ones. And while it often causes us heartache, It is also one of the kindest we can make.
She may recover from surgery quickly to only find that the cancer had already spread. Itís something you have to be willing to risk. Talk to your vet, ask for options, and get his opinion. Be clear in what your expectations are, tell him you want to give her quality of life, not quantity.
Sadly there isnít much any of us can do to tell you if itís the right time. Best I can tell you is to talk to her. Look at her condition, is she still enjoying life? Look at the things she loves, is still doing them. Look in her eyes, often you can tell when its time.
If you donít opt for surgery, take it one day at a time. As long as she isnít in pain and is enjoying life, enjoy the remaining time you have together. Make a list of things she enjoys. Review that list regularly and when you start ticking those items off, youíll know what to do.
No matter what you decide. Good luck. I've been in this position more times than I care to think about. Even when its very clear it is the right decision to make. It still isn't an easy decision to make.