I've heard from a lot of people that seven to eight hours of sleep is best for a full nights sleep. My question is this though, for a person with a disease or some sort of illness that affects their body in some way, what would be their normal sleeping habits? Would seven to eight hours still be enough or would they need more depending on their personal condition?

Your body will tell how much sleep you need a night.

Seven to eight hours is now considered something of an average, though some people can get by fine with six, and some people need as many as ten to function optimally. I would say though, recovering from any sort of physical illness, seven to eight may not be adequate.
I totally agree with Wolfboy, your body is your best judge.
Remember, before the advent of modern lighting, people often went to bed shortly after sunset, and awoke shortly before sunrise. Our bodies are still largely accustomed to this cycle.