A good quality set of bed sheets that are well taken care of should easily last 6 to 8 years or even as long as 10 years.
After 10 years the bed sheets no matter how good of quality should really be replaced since they're likely at the end of their life by 10 years.
10 years is as long as I keep a mattress too so if I had my sheets for 10 years and my mattress for 10 years then i get a new set of sheets and even comforters to go with my nice new mattress.
In fact after 10 years I just get a whole new bedroom set just to have something nice and I can afford it anyway.