I had this decision with my elderly Maine Coon in March of this year.

Once his (probable) cancer had been diagnosed both the vet and myself were concerned with quality of life. As soon as he showed signs that he had 'given up' we decided that that was the time.

I was very concerned that he should not suffer any pain, though it can be hard to know whether a cat is suffering in that way.

When we finally decided the time had come, it was something the whole family concurred with and every one of them came to say goodbye. Even my son who was 300 miles away came home.