When do cells do mitosis?

Answer 1

Although it varies depending on a cell (some cells never undergo mitosis), a typical cell goes through mitosis when all its organelles have been replicated, its size doubled, and its DNA copied. When all this is finished, the cell condenses its DNA and begins prophase, the first phase of mitosis.
A typical cell is in mitosis only about 10% of the times, because the rest of 90% the cell is in interphase, where it goes through normal cell functions instead of dividing.