Final answer:
Cells spend 90% of their life in the phase of the cell cycle known as Interphase.
Step-by-step explanation:
The phase of the cell cycle that cells spend 90% of their life in is Interphase. Interphase is the longest stage of the cell cycle and consists of three phases: G1, S, and G2. During interphase, the cell grows, carries out its normal functions, and prepares for cell division by duplicating its DNA.