When should I change my tires?

I took my car in for an oil change today, and they did an inspection of all the fluids and stuff. The guy said my tires were at 50%. He said I didn't need to change them just yet, but I should keep that in the back of my mind because I would have to do it relatively soon. So how soon is that? How do I know when they really DO need to be changed?

(I put a LOT of miles on my car, so it's important to keep it in good shape. But I don't want to waste $500 if I have a lot of wear left on them)When should I change my tires?
';Got a Lincoln penny? If you do, you've got the perfect tire gauge. The ';penny trick'; works because the distance between the rim and Lincoln's head is 1/16 of an inch -- the minimum required tread depth. When your tire tread is lower than 1/16 of an inch, your vehicle can develop handling problems.';



http://www.nascar.com/2007/auto/cct/11/20/car.care.penny.test/index.htmlWhen should I change my tires?
When you get to the wear indicators, it's time.
yes there should lines kinda like lower than the tread that run cross ways when you get to them its time or you can check them with a penny you put the penny down into the tread and if don't cover like half of ole Abe's head then its time
If you live in an area where it snows during the winter, buy a new set before the snow season starts for safety reasons. You can technically wait till you get to the wear bars on the tire. If your about half through the tread, than double the mileage that you put on them to get an idea how much more time or mileage you have.
Just keep an eye on the wear indicators