Sorry you're having to make this decision. It's the hardest, most painful thing and the only reason I never want to have a dog – knowing this day is gonna come and it will tear my heart out all over again.
It's such a personal thing, specific to each and every dog based on their quality of life, how much they are able to do what they have always done and found comfort from doing and it also depends on how you personally feel and where you are with it too.
For me the decision to call it with my dogs is always based on them, what's right for them and I know is in their best interests which sounds obvious but it's something I tend to do in an automatic, default hard-faced sort of way which can make me seem or come across as quite cold and hardfaced.
Many of us understandably feel a sense of reluctance and guilt / worry they don't have the right to decide, that their pets will let them know and nature will take its course etc but really, that's something we do to ease the burden somewhat. I get it - 100% I get it but I also feel personally this is the one time that our dogs need us more than they ever have.
Pets seldom have a natural peaceful death in their sleep the way most of us would like to think. They might die in the night whilst we sleep but for them the reality isn't necessarily so and if we can let them go peacefully, without any pain or distress, we should.
They have given us so much and ask so little in return but this one is the big one. They need us to do this for them and let them go peacefully which is one luxury and privilege we do have i.e. the ability to just let them go to sleep and not need to suffer.
This general quality of life assessment can be really helpful in just noting and thinking about what's right.
I am sorry you're in this situation and feel for you. It is the shittest, most painful and unfair thing in the world xx 
cloud9vets.co.uk/vet-services/quality-of-life-assessment/