Waiting for a hour would drive a normal person crazy, but Ellen is no normal person.
Her plan is to wait until Viola dies of her wounds. Ellen knows that it would take a long time, but it was better than being in a death trap in her mind. Just entering the house because it was better than pure boredom is just crazy.
Ellen was able to spend her whole time here, thinking. She've a lot of time to think about.
Her past, her plan, the cat, and her new future ahead of her.
Her future was the one that filled her mind. She was lucky that Viola has such a kind father, especially since he left her that message after their fight, but anyone can easily be a better parent than her own. Still, that means that Ellen would have no desire to kill Viola's father. She can have a happy life now.
No magic house. No magic cat. No magic in general. No people haunting her mind. No more feeding the house. No more killings. Well, after Viola, there will be no more killings.
Ellen can have a normal life with an able body and a wonderful father.
She can be happy.
That's how she waited and waited, thinking about her future and her new happiness, though she does admit to reaching over and plucking off some of the flowers' petals here and there when thinking to herself became to unbearable for her.
She smiled when she noticed the big rose bush ahead disappeared.