There are two ways to look at this question. How often do you need to bathe your father, and how often should you bathe him?
I'd recommend doing some kind
of freshening up every day, but a daily bath usually isn't necessary -- and sometimes it isn't even recommended. If your dad doesn't like being bathed or he has dry skin or skin allergies that are aggravated by soap and water, bathe him less frequently. A full head-to-toe bath once or twice a week should be fine.
Work with your dad's personality, medical condition, and mood. However, at the very least, you should aim to wash a few places every day, including the privates, underarms, and between any folds of skin.
What's most important is that your dad stays relatively clean and that he's comfortable with his level of hygiene.