The time it takes for a cell to divide and double itself. The doubling time of cancer cells depends on many things, such as the type of tumor, the resistance of the individual's body, and the location in which it tries to grow. A single cell needs 30 doublings to reach noticeable size (1cm)- a billion cells. Cancers vary in doubling time from 8 to 600 days, averaging 100 to 120 days. Thus, a cancer may be present for many years before it is felt.