I have a ridiculously old rabbit doe - I rescued her with her sister when she was 15 months old, and she's been with us at least 11 years, now. Her sister died suddenly a couple of years ago - fine one day, dead the next.
But, it's clear that her time is now coming. She's losing weight, her breathing has become more noticeable and she's keeping herself tucked up inside her shed. Saying that, she's still eating and drinking, and throwing her toys around periodically, too.
Have spoken to lovely vet, who says to take her in for euthanasia when I think it's time - he says there is nothing he can do to stop her wearing out from old age. My dilemma is whether to bring her indoors (I can make her up a space in a dog crate), so I can monitor her constantly, or leave her outside where she prefers (being indoors is unfamiliar and more stressful) and check on her regularly. It might be hours, days or even weeks - she's a stubborn little madam, so she'll fight to the end, I'm sure.
I hate this bit of pet ownership - knowing when it's time is so hard.