How often should a 98 year-old be given a tub bath?

As long as enjoys. If it is not too difficult for the person helping, and someone can stay with the person while in the bath. It is good to do it everyday. Once a day bathing should be fine unless there are accidents which may increase the need for additional bathing. The most important thing is to prevent pressure on any area of the body for extended period and keep the body clean.