My mercier came stock with Kenda koncepts (the base model tire) I got between 900 and 1000 miles out of them.
I bought 2 brand new tires. Serfas Secas.
I find that my front tire last over twice as long as the back. So I sometimes I have to buy just a new tire for the back, sometimes I buy a new tire for both front and back.
Be sure to keep your tires at the reccomended pressure, this will help them last as long as possible. If you are a heavier rider, your tires will wear quicker then a lighter rider.
When I had the Kendas on my bike I was over 200 pounds. Now that I am 30 pounds lighter the same tire just might last me longer.
Road condition plays a factor too.