There are many signs that death is near, such as loosing the desire to eat for example. This is the body's way of "preparing for death" when a person can not to eat when the end is near, while difficult for the family, can be the kindest thing you can do.
no longer metabolize (or digest) food like a healthy person can. Forcing someone to continue eating can actually cause a lot of harm for the person especially if they may be having difficulty swallowing. Often, people continue to eat because they think their family wants them to. Allowing your grandmother to choose
Some other changes you may see in your grandmother are changes in her breathing pattern, her level of consciousness, and so on. You do mention the darkening of her toenail beds, however. Usually dark toenails are a sign of fungus under the nail and you might want to report that to her doctor for treatment. If she is near death or is on hospice care now and at home, the dark toenails really are not a cause for alarm. By themselves, they are not a sign that death is near.
I am wondering if you are seeing changes overall on her feet, including the nail beds. If the feet and undersides of the feet (soles) are turning blue or purple that could be a sign of changes in the skin, such as mottling, which is a late sign of the circulatory system shutting down. No one can accurately predict how long a person has to live, but I've given you a few guidelines. Don't worry too much about the darkened toenails, but concentrate on this special time you have with your grandmother!