In our project, every significant change to the system is reviewed by the team leader or together with another developer who's gonna be the main "consumer" of the new module. We talk on skype and either use Rudel in Emacs (a plugin for collaborative editing, basically it allows several users to edit the same file live), or TypeWith.me (Piratepad), or one of us shares his screen in skype.
It's hard to quantify this, because mundane changes, like new views, pages, etc. are not reviewed. We do review new modules, major updates and refactorings. As for big changes, code review can take from 10% to 30% of time, but it's worth that.
I can say pair programming, when 2 programmers do edit the same file at the same time, not just sit at the same computer, is a lot better than the usual office practice of sitting behind one's shoulder.
For simple things like naming conventions and scope errors we use our own or open source automatic tools (jslint, pylint, pyflakes, pep8). And we don't limit commits and pushes: we use Mercurial which has very easy branching and merging (I have to say, easier than in Git). Bugs are not a code review matter.
We do team meetings where the changes and new things are announced, but there, not everyone really pays attention. Probably we should do a bit more code reviews.