The S&P 500's long-run historical average annual return is roughly 10% before inflation. A higher assumed return reaches your FIRE number sooner; a lower one takes longer.
A higher withdrawal rate produces a smaller FIRE number and reaches it sooner; a lower withdrawal rate produces a larger FIRE number. FIRE number = annual expenses ÷ withdrawal rate.
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Coast FIRE = you stop actively saving but let existing investments grow untouched until traditional retirement age.