The biggest and IMO most important variable is how the shirts are washed. The shirts I send to the cleaners every two or three weeks to be washed go for about 2-3 years before the cuffs and collars are frayed and the color is off (so about fifty outside cleaner washings seems right). The shirts I wash at home in cold water (slimmer shirts where if they shrunk at all they'd be too tight) last much longer-- 5+ years. But I'm lazy and would rather wear out a shirt by sending it to the cleaners than washing and ironing myself.
I have somewhere around 75 dress shirts currently. I wear them 5 days a week and take them to the cleaners after each wearing. I have varying degrees of price and quality, but I can count on one hand the number of shirts I have thrown out because they were "worn out" (fraying collar or cuff.) I have some that I know are well over 10 years old. I dont generally wear a tie so I have a lot of color and plaids. I went through an Etro phase a few years ago. I usually get tired of them before they are worn out. You should be pretty good. I only buy now when the deal is too good to pass up.