I have three N900s. All used. In chronological order...

#1) Obtained in June 2012. No screen protector. Used extensively till June 2013 (i.e. for about a year). The screen had very light marks when I got it. One slightly deeper mark, can be seen and felt when stylus goes over it but does not obscure anything. These are still the same now, a year later.

#2) Obtained in Autumn 2012 (can't remember which month exactly). No screen protector. Screen in pristine shape. Device not used and currently indisposed following a failed flash. Kept for spares.

#3) Obtained in early June 2013. Screen protector on. Used extensively as a replacement for #1 since the purchase (i.e for about a month). The attached screen protector had very light, nearly invisible marks when I got it, which became quite noticeable by now. To the extent that I have bought a set of replacement screen protectors (this one) and am considering swapping it very soon.

Conclusion: A screen protector gets to a worse condition in a month than an unprotected screen gets in four years (3 with the former owner plus 1 with me). If you want to use a protector, use the cheapest you can get since you are going to replace it in a couple of months anyway.