We're nearing that with my wife's mom. Since my wfe's dad passed away 5 years ago, my wife's 88 year old mom has needed increasing levels of help. My wife spends 4 or 5 hours a day with her helping with cooking, housework, shopping, doctor appointments, hair appointments, etc. She definately couldn't stay in her house without this type of assistance but we know it won't get better or easier. My wife's brothers also live nearby and help out around the house as well as taking over when we're out of town.
Fortunately, her mom is pretty easy going. We even take her on one of our vacations each summer to a lake house. We can still get her into a boat and she loves tooling around the lake and watching her grandkids tube, ski and wakeboard. She also loves the evening BBQs on the deck overlooking the lake. We always go with my high school buddy and his family and he takes over entertaining my m-i-l so I can man the grill. Works out great for all of us but I know she won't be able to do this too many more years.
We had my parents to deal with in 2001. My mom had Alzheimers and my dad's health started slipping from the strain (he was determined that he would be the one to take care of her). My dad ended up in the hospital (and spent 5 months there before passing away) and my wife and I took over care of my mom. We did the tag-team thing as my mom could not be let alone at all. It was even tougher because our sons were 9 and 11 at the time needed to be dealt with as well. After a few weeks it was clear we were not equipped to deal with her and moved her to a care facility. My dad passed away the day before their 59th wedding anniversary and my mom went a couple of months later. They were high school sweethearts and we always knew they couldn't live without each other for very long.
It isn't easy but you have to balance what is best for all and not make decisions out of guilt. Sometimes a parent needs more care than you are capable of giving. No matter where they are, make sure you spend a lot of time with them. That's the way you show your love.