I'm so sorry for what you're going through. It's really, really hard, and there isn't any way to truly prepare for that loss. Keep your baby comfortable as long as you can. I agree, they'll let you know when it's time. I lost one of my dogs in his prime to a sudden hemmorhaging tumor and the other i put down at 14. Neither way is preferable or easier. i think I cried more over losing my dogs than when either of my parents passed. Go figure!
Hugs and payers for you & your furbaby!