For most short stays, third-country nationals may remain in the Schengen Area for a maximum of 90 days within any rolling 180-day period. The window moves: from each day of your stay you look back 180 days and confirm your total presence does not exceed 90 days.
Entry and exit days both count. Stays under an EU residence permit or long-stay (D) visa are not counted toward that short-stay limit. The European Commission publishes an official short-stay calculator to check or plan trips against the rule.
Source: Short-stay calculator — European Commission (Migration and Home Affairs)