Which is a thing that depend very much on team culture. In my team it is perhaps 15 min for smaller fixes to get signoff. There is a virtuous feedback loop here - smaller PRs give faster reviews, but also more frequent PRs, which give more frequent times to actually check if there is something new to review.

Does your team just context switch all the time? That sounds like a terrible place to work.

Similar in my team and I don't feel like there's much context switching. With around 8 engineers there's usually at least one person not in the middle of something who can spare a few minutes.

How can everyone be familiar with everybody else's work?

Usually at most 2-3 engineer have enough context to fully understand what your code is doing.

If I'm deep in coding flow the last thing I'm going to do is immediately jump on to someone else's PR. Half a day to a day sounds about right from when the PR is submitted to actually getting the green light