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:
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.