The problem here is not that developers are too busy, but that code reviews are considered second class citizens to churning out new code. It's like saying "many devs are super busy working on feature A so they just write quick and dirty code for feature B". If reviews are integral part of feature production pipeline, there should be no issue to sit down and spend a day reviewing code. For bigger, more complex things it could be a few rounds of reviews.

There is an approximate non linear relationship between time it takes to produce the first PR and time it takes to go through all rounds of review. This time can be pretty reliably calculated and taken into account.