In what mitosis phase does a cell spend most of its life

Interphase is not really a phase of mitosis, it is when the cell is preparing for mitosis. It has three different phases: first gap, synthesis, and second gap. During all three phases, the cell grows, feeds, develops and does whatever it's supposed to do, but only in the synthesis phase is mitosis even recognized. It's sort of like the puberty of the cell. Chromosomes start to devide.