The longest stage of the cell cycle is interphase.
Interphase is the phase in which the cell spends most of its life. During this period, the cell grows, develops, and prepares for mitosis. It can be further divided into three sub-phases: G1 (Gap 1), S (Synthesis), and G2 (Gap 2). In G1, the cell grows and synthesizes proteins necessary for DNA replication. In the S phase, DNA is replicated, resulting in two copies of each chromosome. In G2, the cell prepares for mitosis by producing the organelles and molecules required for division. This extensive preparation and growth ensure that the cell is ready to divide successfully.
In contrast, prophase, metaphase, anaphase, and telophase are all part of the mitotic phase and occur in a relatively short period compared to interphase. Therefore, interphase is the longest and most crucial stage of the cell cycle.