It really depends what sort of riding you're doing. Having suffered a lot of punctures last year I bought myself some Specialized Aramadillos this year for the winter bike and Continental GP3000s for the summer bike. I am doing around 500 to 600 miles per month, roughly split between the two bikes, and so far have only suffered one puncture since replacing the tyres in, I think, March or April. Neither tyres are showing any great visible signs of wear, so I think it likely they will do a couple of years, which is not a bad mileage. I also bought a track pump this year, which means I can now get them up to their recommended pressure of 120 p.s.i., and I think this is helping. My previous experience would suggest that tyres start to puncture on a regular basis before the tread looks totally worn out, but this may not be the general trend.
I would certainly endorse going for the best tyre for your conditions rather than choosing one for its reflective properties. You can put reflectors anywhere on a bike. Speaking as a driver I think the best place for reflectors is around the ankles or on the pedals/shoes, as seeing those legs pumping up and down is the best indication that you are approaching a cyclist.