First, I don't think you should wait and I'm not sure you'll know when the right time is. My 16-year-old cat almost died on my kitchen floor in March because I couldn't decide. He decided for me, and I did take him in quickly and have him euthanized, but I am so sorry now that I let him suffer for so long. It was needless. And again, I couldn't decide what was the right time.
I feel your pain. I just put my 19-year-old cat down last week. She was hyper-thyroid, had it removed, it grew back and she was so skinny and her fur was matted and she had arthritis. But she could still meow, walk around, etc. I just couldn't decide.
My sister-in-law, who is a vet tech, came to visit last month and saw her and said to me "why are you doing this to her? She gave you 19 great years, and you need to give her a peaceful way out. Don't let it go until she is dying on your floor and it's a crisis and she is in pain."
In fact, she convinced me she probably was in pain and uncomfortable. And who wants to live like that!
If she isn't making it to the litter and her legs give out, and she can't jump and she has arthritis, imagine how that must feel to her. Her quality of life is gone. It is up to you to help her not suffer anymore. And you can do it.
I was in the same position as you until my SIL talked some sense into me. I picked a day (that was hard) and took her in and let her peacefully go. It was the right decision. It was hard, I cried and doubted myself. But I'm SO GLAD she isn't in pain anymore and I didn't have to watch her die of heart failure or let her die alone in a corner somewhere.
You can do it. It's for her not you. You are keeping her alive for YOU. That's what I was doing. The best thing for her would be to end her suffering. Hugs to you. Such a hard decision and I wish you the best.