Got a 'we'll be in touch' after a strong second round. How soon is too soon to follow up?

It's been nine days since a strong second round. I sent a short thank-you the next morning and nothing since. The hiring manager seemed genuinely into it on the call, so I don't want to disappear. I've heard 'one follow-up a week, max' but that feels arbitrary. Does anyone have a rule they actually use?