This is typical infant behavior. Keep in mind infants typically don't know what their hands are doing, that they can control them, or that they are even part of their own body. (My 5 month old son still wakes himself in the night, by whacking himself in the face -- he thinks someone else is doing it! I have to tightly wrap him in a blanket, to jeep his hands away.) Be sure to keep her fingernails filed down, so she doesn't hurt herself. My first two kids hated pacifiers, but my 5 month old likes them. See if she will peter a pacifier to her hand.
Wen you're ready to stop the habit (probably in a boutabout a year), simply rub the cut end of an artichoke stem on the offending hand. Raw artichoke tastes AWFUL. The bad taste will stay on her hand (and in her mouth) for some time, doesn't stain, is a natural, and won't cause a rash or allergic reaction. Most kids try it only once or twice, before they get the clue that putting a hand in their mouth isn't a great experience.
If the behavior persists, wait until she is old enough to learn to stop. 18-24 months old should be old enough -- girls are pretty smart and learn fast at a younger age.
I assume this is your first child. Don't spoil her, even though you'll want to!
