When do you know it's time?

My wonderful, sweet little cat is getting so frail and old (she is 15 years old). Her movements are very slow and calculating. Every step is carefully calculated and placed just so. I suspect her arthritis is making her so careful in her movements. Her previous vet said she had 'some' arthritis in her back. That was two years ago. I think it has gotten much worse.

She has become a very messy eater, sometimes getting food all over her little face. Once a week or so she messes on the floor in the bathroom near her box.

B says we should just have her put down (he is not a 'cat person' nor a very sympathetic or compassionate person - he never really loved an animal).

I don't mind cleaning up after her. If she were in obvious pain or could no longer control her bowels then the decision would be easier.

But for now, she is just old and slow. She still purrs as loud as ever. She still loves to be held and will sleep all night in my arms. In my eyes I see the young Gemma. I love my little cat and will miss her when she is gone.

But I don't want her to be in pain.

So, really, how do you know when it's time?