We have a nearly 12 year-old greyhound who is very much loved. We brought her home from the retired greyhound kennels 6 1/2 years ago, at a time when I thought I'd never be able to have children. She helped to fill the massive hole in our home and in our lives.
Fast-forward to now, and she is undoubtedly deteriorating. She has lost quite a bit of weight and muscle tone. She sleeps most of the day (even more than normal for a greyhound!) and she can barely tolerate walks. She's enthusiastic about going, but starts to limp pretty quickly, and gets breathless. She's had days recently where she's hobbled from bed to bed. Her back legs are definitely getting weaker. It's been over a period of a couple of weeks really. She fell off her bed the other night when she tried to get off (she sleeps on a sofa) and just lay there on the floor 
I took her to the vets last week, and we came away with some metacam and some tramadol. I thought the metacam perked her up a bit the first few days, but she's not had a good day today. She's barely eaten (picked the nice bits of tinned dog food out and eaten that, not eaten much of the dried food). She has limped around a bit. Sometimes she seems happy just pottering around the house and sleeping, other times she doesn't. We're not keen on putting her through further investigations - what's the point?
I know that she's old, and getting frailer, and I know that at some point the time will come where I have to make that decision, and it will be my decision, I'm sure. My husband will leave it up to me, I know. I just don't know when to do it. It kind of feels a bit early now, but I don't know if that's because I'm not ready to let her go - I've cried so much thinking about losing her. But I don't want her to suffer. I don't want to leave it too late. I haven't tried the tramadol yet, but maybe I should.
How do you know when it's time? I'm also aware that I'm around a bit this week, and less next week when I have to go back to work. She'll be on her own more then. I don't want her to get into trouble when nobody's here.