Cells divide at different rates. Some of your cells, like the ones that make up your muscles and brain, don't divide. Others, like the ones in your mouth and skin, are always getting replaced with new cells. The actual cell division may take only about one and a half hours, but in between, the cell has to copy the DNA (chromosomes) and make new cell parts. This can take about a day for some cells and many days for others. There may be cells that divide even faster that I just don't know about.