The thing in this movie that is not established very well, is that Grace has been on Pandora for many years.
We do see glimpses of this: there are pictures of her avatar interacting with Na'vi children, and if I remember she also helped to set up things like schools, and taught some of the natives English. Clearly this has taken quite a bit of time, and that is along with research into the natural things on the planet.
Being on Pandora, the only time that she can actually go outside is when she is in her Avatar form. Otherwise she is confined to be indoors, unless she is wearing a bulky gas mask.
Her Avatar is also incredibly strong and agile compared to her normal body. Her human body is aging and decaying, whilst her avatar is still much the same as it was.
All of these things combined, it is not hard to imagine that when she isn't in her avatar, she is probably quite unhappy. As a human, she has had to stay inside for so many years, feeling weaker and older as time goes on, and not being able to interact with the people that she has grown to care for. This is probably quite depressing.
We see Jake change over the months that he is on Pandora, and he doesn't want to ever leave his avatar. He is forgetting to eat and rest. Imagine that going on for years and years.
Grace is probably always anxious to get back into her avatar form, but knows that she can only do it for so long before it starts acting detrimentally to her. It's basically her addiction. She has to manage her time in the avatar body, in order for it not to overwhelm her.
One of the problems with the film is that it doesn't establish Grace's character very well, as she is probably one of the most interesting characters in it.
