Just to point out, there are 2 general types of kidney failure, Acute & Chronic.
Acute (which it sounds like Penny has) is usually caused by infection or poisoning. Although it can be dangerous, if treated in time, the kidneys can recover 100%
Chronic is the more common type, this is irreversible and the kidneys deteriorate over time. It can take from months up many years. It can be slowed down with diet and/or meds, but it is a case of "how long is a piece of string" timewise. We lost Clapton at the beginning of October, and it would have been 5 years since diagnosis had he made it to the end of the month.
Another poisoning culprit seems to be the clear type of antibacterial kitchen spray, ie Dettol spray - although supermarket own-brand seem to be the same. A few have warnings on but not all. I think people assume as they claim to be safe to use on high-chairs etc, that they are non-toxic. They are safe for humans, but cats are unable to break down the toxins so they build up over time. Also a rule of thumb is any disinfectant/antiseptic that turns cloudy when water is added is also poisonous to cats - it is the phenols that poison them.
As for lilies, anything in the lily family is poisonous, including onions, garlic, leeks. I think lilies are more toxic though - any part of the plant is dangerous.