What kind of tires are they? What is your driving style?
M359s are what, around $1200? So you paid around $700 for 4 tires -- if so, that actually is pretty "cheap" for a full set.
ETA: to answer your question I have Toyo Proxes4 with around 10k miles on them and they have plenty of tread left. No signs of wear at all, really, even on the rears. But I missed track season last year, so I'm sure I will eat through these this Spring