It's never easy losing a cat
You'll feel when it's time, if she's not enjoying doing things any more that's a clear sign that she's ready to go
I had to make that decision for my angel Ebony a few years ago, on her last night all she could do was walk in circles and she was no longer aware where she was putting her paws, cancers was taking over her body, she went into the vet for tests to find out what was wrong and they found all the cancers in her, they think it had got to her brain as well by that point
My angel was only 7 but she'd had a horrible time of it before she was rescued, and had a lot wrong with her even before the cancers