An app I submitted a while ago to testflight changed to "in review" a couple of days ago, and is still in review. Is this normal to be in review for this long?
Yes, apps can be In Review for a number of days, regardless of whether it is a TestFlight app. For what it's worth, I currently have an app with a status of In Review which has been in this state for over 7 days (excluding Waiting for Review time).
Typical review wait for an update to a released app is about a week, and then the review itself is typically less than a day. I have no experience yet with the beta review.