The rule in one sentence
You can spend up to 90 days in the Schengen Area in any rolling 180-day period.
The phrase “rolling” is the part that confuses people.
Why “rolling” matters
It means the window is always moving. At any date you check, you look back 180 days and count how many days you were present.
What counts as a day
In most interpretations, any day you are present counts as a day, including partial days.
How to think about it
Instead of asking “How long can I stay this trip?” ask:
“How many Schengen days have I used in the last 180 days?”