How long do you think code reviews should be? Ours can run from anywhere between 10 minutes to 4 hours

Well it shouldnt be a meeting for a start.

As long as you need to review it. Depends on how much it is and how complex, and whether it's enough to review diffs or a full review is required.

Also, what's the purpose? Checking that style guides were kept? Or do you want to find bugs?

my team(4 of us) has code review daily meeting first thing in the morning. we briefly show and comment what we did yesterday. there are questions and discussions about why did we wrote code that way and if we could do it better, constructive advices from colleagues is common thing. it lasts about 20-40 minutes.

for detailed code review we use pull requests that team lead and other web dev on project go trough all the code. we use PR on several projects, on some older projects that we maintain we don't use it at all. it is all matter of agreement!

People make actual meetings for this?
Just make a pull request and let someone read over it.

This kind of task should take less then an hour at worst unless you’re changing a lot.