Tiger Is Dying. When?

My 18 year old kitty is dying of kidney failure. Her lab values are through the roof, she's less than 5 pounds now, she wobbles when she walks, and now she barely eats. I've been giving her SQ fliuds each day for about a week. She can't seem to get her tongue to sit right in her mouth and fiddles with it. All of this has transpired within the last 2 weeks.
This is heartbreaking. I've had cats and dogs all my life, but you know how every once in awhile a special one comes along? Well, Tiger is special. She follows me everywhere, and talks alot. She's only content to sleep at night if her paws are resting on my hand. She gets in my face, tickles me with her whiskers, and meows till I hold my hand out for her to put hers on. She plays with a little toy on my bed just like she was a little kitty. She would race around my bed in circles and pounce on imaginary mice to make me laugh. I have 3 younger cats that like to play too, but the speed of Tiger's paws is twice that of the other cats. She was so fast my daughter and I jokingly call her Miss Miagi (like from karate kid).
Now, I'm tortured with when/how to decide to have her put to sleep. Until today she seemed comfortable. Today, she's acted restless in spite of her decline, wandering the house, so I think she's uncomfortable. My sister, who grew up with Tiger as well, describes her as not a regular cat, more like a funny little fairy or angel. Letting go is just horrible.