Although it shouldn't happen (sheets should last years I think) I couldn't bring myself to complain after 2 years.
I think M & S standards are much lower than they used to be. I recently bought their Supima sheets and the stiching is coming out of them already! I also think one of the sheets (the base one) is not Supima as it feels completely different to the other sheet and pillowcases. (I'm taking them back to M & S when I get the chance but I don't live near one)
Think I'll use John Lewis for sheets next time I need some.
I noticed one of our 4 year old M & S base sheets had a big threadbare patch this morning. We change the bed about fortnightly and rotate the sheets around so that each one won't go on again for a couple of months so its not as if it has got a lot of wear and it is a more top of M nS range sheet! What would you do, do you think its worth complaining to m n s or just put it down to experience