I know exactly what you are going through. Everyone told me I'd know when the time had come with my cat, Taz. I would always second guess myself and wonder if it was, then say oh, no it's not. But the last time I took him to the vet I sort of knew it was the last time. And when the vet told me she wouldn't argue with me if I decided to let him go right then, I knew it was time. (oh geez, here are the tears again, over a year later)
Like Scruff is to you, Taz was my baby boy. It was the hardest thing I have ever done, but I do know it was the right thing to do.

You will know (deep down, even if you don't realize it) when it is time. And when that time comes and I read it here on Pet Talk, I will probably cry for you just like I did when I said goodbye to Taz (and like I'm doing right now).

I do hope you have several more months with Scruff though.