My mada has 25,000 miles with the original Conti's. At what time did you guys replace? I've heard of using the old coin + tread trick which by that it looks like it's time to replace but what other method's did you guys use?
Plus what do you guys think of the Bridgestone Dueler HT's? I've read on here that people like the AT's but how quiet is the ride?

There are several threads on recommended tires.

Check the forum here and you will find more than you may want to know.
http://www.clubarmada.com/forums/forumdisplay.php?f=9

Tire wear really depends on you and your driving style. However, when to replace them is pretty simple. If you use the penny trick of putting Lincoln's head in the tread and you can see the top of his head, time for new ones.

I took my OEM contis off at less than 3K and the Coopper Zeon Tires I have have over 43K on them and don't need to be replaced (knock on wood) yet!

Tires also have the wear indicators. Its a little raised sections in the groves of the tire. You should replace them when the bars are to the surface.

When did you replace tires?
As soon as possible. The Conti's are so bad, they should be just thrown out upon delivery. I have mine in my garage with 1500 miles on them.