Sadly, the necessity of the screen saver is a modern urban legend. Your computer's monitor is not going to etch the last thing you saw directly on the surface of your CRT unless you try beyond all reasonable measure. The appropriate statistics:

A monochrome (black-and-white) screen takes three to four months with the same image before ill effects are obvious. This is actually a little bit evident; on my old Macintosh SE, the menu bar is slightly visible, even when it's not on the screen. But that's after 16 years of constant use.

A color monitor will need over four years of nearly constant use before anything will happen; that's with constant screen imaging, twenty-four hours a day. Not too likely.

Note that I advocate the use of screen savers regardless; though they may be a myth, screen savers are still fun.

Information from Macworld Secrets, 1995 and the Macintosh Bible, 4th Ed.