Ah congrats! I recently accepted an offer there in November and will be starting in January. I'm a new grad!
Here was my timeline -
1) I interview
2) Got a call from HR the same day I left telling me that they would like to move forward with the process and basically offering me the job but I just needed all my references completed first
3) I made sure all my references were completed within a week and at the one week mark, I got a call from HR again officially offering me the job!
4) Right after we talked on the phone she emailed all the onboarding information, links to sign the electronic offer, etc.
So overall, it took me about one week exactly from my interview date to get a written and signed offer! What floor will you be on? Do you know your start date?