How long should I wait for a response from my potential reference regarding contact info? (Company is waiting to hear from me)

Last Tuesday (6 days ago), I got an email from HR at a small company after going through two interviews for the position. They wanted 2-3 references from past supervisors/professors. I emailed my past internship supervisor and former professor on the same Tuesday. My past supervisor got back to me on Wednesday, and my professor responded to me on Thursday. I asked my professor what form of contact would be the best (phone or email), but she hasn't responded to me since. I'm worried that I'm making the HR person wait too long (I followed up with her on Friday with just my past supervisor's reference and that I was still waiting on my professor). I emailed my professor again today as a reminder as well. This would be my first job after graduating college so I'm not sure how long is too long to ask HR to wait. Thanks in advance!!

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