Wait until you start flatting to change them, or if you feel like it get new ones. Myself, I put a new chain, cassette, and GP4000 tires on every 3,000 miles regardless (if they all make it that long). $100 every 3,000 miles is cheap for a hobby, in my opinion.
On the same token, there are people here who will routinely get 5,000 miles on a set of tires, so your budget, and preferences will make the call. It's not a matter of safety or requirement.