I have a asus monitor that is around 5 years old I use it heavily for gaming movies and browsing. I also had a viewsonic that i bought in 2005 that I gave to my dad and its still working so they can last awhile or im just lucky.
It's probably more a question of how long will the hardware and specs be relevant.. Next thing you know it's going to be .00001m response time an 5,000,000hz monitors (Clearly exaggerating)