AFAIK in most companies code review is done whenever a task is ready for it and my current team is not an exception. What differentiates my employer is that there are many cases where you don't really have to pass it unless you really want others to take a look at your code.

It depends on several factors:

1. How many people in your team have understanding of that particular part of the system.
2. How many of them are available for code review at the moment.
3. How crucial your changes for the system as a whole from architectural stand-point.

But most of the time I still ask others to take a look, unless I know for sure that my colleagues are busy and the code is a bugfix for something I've done myself.

In the company I work for, we do code reviews after every ticket a developer completes. So when I finish one of my tickets, another developer takes a look at my changes before they will be merged.