The test itself is 134 minutes: 64 minutes of Reading and Writing, then a 10-minute break, then 70 minutes of Math. But plan for roughly four hours at the test center once you add check-in, seating, Bluebook setup, and dismissal.
The gap between 134 minutes of testing and roughly four hours on site is what catches families out, particularly when arranging pickup. Plan the whole morning, and eat beforehand on the assumption that the 10-minute break is the only one you get.
The full picture, with the surrounding detail, is in SAT Test Day, Hour by Hour: What Actually Happens.