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The official rule for the Schengen visa is that, on the day you first
enter any country within the Schengen zone, you start a 180 day
"period", within which you can stay a maximum of 90 days in all
Schengen countries. After you have racked up your 90 days (in one go,
or in several chunks if you leave to non-Schengen countries & come
back), you can't enter any Schengen country again until your 180
period ends. And on day 180, no matter what, you have to leave
Schengen territory (even if you haven't used all your 90 days), but
you can then turn around & come back in again the next day & get a new
90/180 day period. So: if you enter on day 1 & stay for 90 days
immediately, you have to leave on day 90 & cannot re-enter any
Schengen country until your 180 day "period" is up.
Espero que isso seja útil para você.