this question came up about 2 weeks ago and got lots of replies. check this and see if it helps:
http://bike-pgh.org/bbpress/topic/when-to-buy-new-tires
2010-10-31 02:37:29
9
When to buy new tires
I cycle about 5000 miles a year,biking a lot in the spring and summer,and tailing off in late autumn, and not at all in winter.I haven't changed my tires for 3 years,but they still seem to be holding up pretty well.Is there any set rules of when to get new tires?? If I had knobby tires I could probably tell better when I need a new set,but I have smooth road tires on my bike.Any opinions would be appreciated.Thanks.
2010-10-30 22:45:36
I'm no expert on this particular subject, but I'm also a big fan of the "if it aint broke, don't fix it" policy. Until The tires are either badly dry rotted or severely worn, why replace them?
2010-11-02 01:03:20
15,000 mile on a set of tires? I am guessing I put on around 5000 miles a year and need to put new tires on yearly. Is this because I buy inexpensive tires?
2010-11-02 16:39:27
is there a wide flat section in the middle of the tire? if so its worn out and should be replaced. your tire profile should be nice and round.
2010-11-04 14:39:02
I use my tires until I see threads- they are definitley on the far side of worn out before I replace them.
2010-11-04 16:16:43
If your tires aren't round, you need to corner more 
2010-11-04 16:31:47
On road tires, the tread texture doesn't really do very much, hence completely smooth high performance tires, so tread wear isn't a good indicator. As the tread gets thinner, it will become more likely that you will get flats. I usually wait until the rubber down the center looks visibly thinner, and get a flat and can feel that it's very thin, or until I can see the fabric.
If your really compulsive you could just let out the air, take out the tube, and feel how thin the tread in the center is, you'll be able to feel when it's paper thin.
2010-11-04 16:40:31
I don't think there's any argument that if, after a flat, I can see light looking from the inside of the tire, it's time to change.
More I worry about hard banking on a turn. Do I have less traction with a tire with some dry rot, and if so, how much less? The 2nd last thing I do after a multi-dozen-mile ride is to transition from a fast downhill going east to a stiff uphill going north. The distance I get without having to pedal is directly proportional to the speed I dare take that left. That in turn is dependent on how much I trust my tires.
2010-11-04 18:47:44