The round
Sixty minutes on a blank board is not a test of what you remember. It is a test of how you decide, in an order the room can grade. Here is the order, minute by minute, and what to do when you fall behind it.
How long is a system design round, and does the method change with the length?
Forty-five to sixty minutes, and the method does not change; only the depth per phase does. In forty-five, requirements shrink to five minutes and you commit to a design faster. In a ninety-minute onsite you get a real second deep dive and a day-two pass on resizing and deploys. The five-phase spine, requirements, options and commit, the spine and data model, the deep dive, failure and recovery, stays in that order under any clock.
What is the interviewer actually grading?
Three things, and every phase spends one of them. Whether you can decompose a vague prompt into requirements and boxes. Whether you can justify a choice against an alternative and say what it costs. Whether you can reason about failure: notice it, then recover. A diagram that is elaborate but spends none of those is not an answer.
What do I do if I am behind at a checkpoint?
Cut scope out loud. At minute 10 with no numbers, stop clarifying and ask the three scale questions. At 20 with no commit, commit in one sentence and fold the alternate into what you would revisit. At 35 with no data model, draw the entities onto the diagram and skip the third walk. At 50 with anything unfinished, drop it mid-sentence and protect the failure minutes. Naming the cut is itself a senior signal.
Should I ask questions or start designing?
Ask, for the first ten minutes and not a minute more. The interviewer is deliberately holding information back, so a detail they volunteer is one you are expected to use. When they say to assume any scale, state a defensible number out loud and confirm it. A silent assumption cannot earn credit, because the reasoning was never heard.
How much should I talk while I draw?
Narrate the phase you are in, always. "Let me spend two minutes on requirements first." "I am going to commit now." "I am watching the clock, so I am moving to failure modes." Managing the clock out loud is part of what is graded, and the sentences are short enough that they cost nothing.