On our team we review feature branches. When the developer thinks it is ready they have an assigned reviewer. The reviewer makes comments, the developer responds, and they go back and forth until it gets to LGTM. My general rule is that I need to understand what the branch is doing and why, so the review takes as long as it takes. If it takes too long that is often itself a sign that comments or documentation is needed.

For interns, they might not know when to ask for review, so you might need to check in on them once a day.