How do you know it's time?

Gonna keep it short and sweet. Our puppy (10 but always a puppy) was diagnosed with lymphoma about a month ago and it's been a really rapid decline. We aren't really in the market for chemo or treatments like that...

How do we know when to euthanize her? She started slowing down with the eating, so now we're just spoiling her with fast food for dinner, she's not really walking, but she still wags her tail and comes looking for attention... how do we know if she's suffering or if she still wants to live?

I just don't want to take life away from her if she isn't ready, but I don't want her to suffer either. Is sooner really better than later?