From many medical students, I was told that most students take the USMLE Step 1 after their second year of medical school (summer, usually June). Some schools require that students take it and pass it before they continue with the rest of their medical education or pass it before they graduate.
At my school, we have until three months following the last day of class of our second year to take the Step 1 for the first time. We're allowed to go ahead and begin our third year, even if we fail the exam, because as long as we pass it in three tries, it's all cool. Fail three times and you're dismissed.
We had a few weeks to study for it in May and June. We could take it any time during those weeks, as long as we were done by the beginning of third year (June 30th). If you later found out that you failed, you had to complete the rotation you were currently on, but then take the next one off to study and retake the exam.
