It’s variable, by design, and takes into account your “trust level” and the “trust levels” of users flagging you. A well-established, active contributor might have enough weight to instantly hide someone’s first post with a single flag, but an active contributor’s posts will take a lot of flags to hide. Even a single flag goes into the moderation queue, regardless of whether that flag automatically hides the post or not.
When enough flags accumulate to hide a post, that post is displayed under a fold, so that people won’t see it unless they choose to do so. The poster gets a notification, and until a moderator makes a decision, they can edit their post and this will cause the post to be unhidden (it can be flagged down again). If the mods disagree with a flag and overrule it, no amount of further flagging will hide the post, but the mods will continue to get notified, and might take action against the people flagging it if the flagging is frivolous or an attempt to censor posts that don’t violate any rules.