Apply when your application is genuinely ready, not simply when your friends or classmates do. The full cycle runs about 15 months end to end: AMCAS opens in May, and matriculants start medical school the following summer.
The real readiness test has four parts: your MCAT is done and reflects your actual ability, your clinical and service hours are substantial and consistent, your letters of recommendation are lined up, and your personal statement is genuinely one you'd be proud to have an admissions committee read. Applying while only half-ready usually just means reapplying a year later with more time lost than gained — reviewing your full readiness with an advisor before you submit is far cheaper than finding the gaps out after a cycle of silence.