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.