Only about 2.5 for me (at my font size on a 1920×1080 screen), but it's a productivity win regardless, and it's a massive productivity sink when I have to work with code written without that constraint.
An 80+ character line is also a code smell for me; if a statement is so complex that it pushes past the 80 character mark and it can't be split into smaller chunks on their own lines, then I know for sure that something needs refactored (whether it be the statement itself or the nested control structures around it).
Yeah, I actually keep my editor columns at about 65-70 with :set nowrap. I find this gives me appropriate pressure to write shorter lines, but if I go past the end I've got another 10 columns or so until I hit 80.
