Look in Windows / Preferences (at least on Windows – IIRC it moves around for different operating systems) then:
General -> Editors -> Text Editors -> Show Print Margin
Tick this and it should show the line.
As a quick way of finding this, use the search filter in the top and filter on “margin”.
Notes from the comments – unverified by me, but I have no reason to doubt them:
It has changed somehow in 2016: For details see [here] (https://bugs.eclipse.org/bugs/show_bug.cgi?id=495490#c2) You have to set it in the formatter: From menu [Window]–>[Preferences], select [Java]–>[Code Style]–>[Formatter], and then edit your formatter profile. In the tab page [Line wrapping], you can find a setting named “Maximum line width”. Change this setting, and the print margin in Java source editor will be changed too.