Revisions

When do you think revisions are enough to warrant shelving the book? I recently was told my by Crit buddies that my book is not believable because of a few character flaws, or things that aren't consistent enough, which doesn't bother me that much, but to implement the changes will require some big rewrites of the material.

So when do you decide that it is time to shelve the material and start from scratch on a new book?