When will she tell me that she is ready to go to Cat heaven?

yes,but you are fortunate she has lived that long.her time probably is close.My favorite cat was 11-12 yrs & pesticides severely weakened him & bad guys killed him Jun 1st with deadly crap in my carport where I had 2 cats(one survived & is scared & mad at me,thinks I am the one who did it) my favorite one(a very special one) didn't survive & like loosing best friend.I have lost 7 dogs in almost 5 yrs the same way.Hope you can spoil her as long as you can & make her feel comfortable.I no0w have a little kitten in my house,was wild & I finally made friends but not house trained yet & flees but too small for poison treatment & breathing problems from chems gvt is turning loose constantly.He is a different breed?face looks like a bobcat & loveable as all getout.,but miss my cat like as if he were a kid.