how do you know when to replace a chain ring?
 

Would you be able to tell just by riding it?

Can you tell just by looking at the patterns?

If the teeth are sharp and pointy, it should've been replaced already.

I just retired an outer chainring on a pair of Dura-Ace cranks I've been running since 1996. The only reason I decided to do that was because chain suck occurred a couple of times shifting from down from the big ring.

There's no need to replace a chainring (other than to change size) until there's a need to do so. If the chain is running fine, there's no suck, or slippage under power or any other specific mechanical issue you can ride the ring until there is.

My old road bike has over 50k miles on the original rings, the other of which any Ninja would be proud to own, but they still run fine.

advantage: steel chainrings.

It depends on how good you are about replacing chains before they're too "stretched." If you're good about changing them at or before the 12-1/16" mark, the rings will wear like this, and you'll be able to use them for 100,000 miles or more:

http://home.comcast.net/~carlfogel/download/zzz_006.jpg
(Jobst Brandt's chainring.)

But if you aren't as vigilant about the chains, the teeth will wear to points, and may skip under load.

http://home.comcast.net/~carlfogel/d.../wornteeth.jpg

Either way, you'll know to change the rings when they begin to skip.

those are 7075T6 alloy too .. low cost aluminum chainrings are even softer..

Well, if Al rings last over 50,000 miles as FB reports and I've got a Shimano 6503 triple crank with 35,000 miles and still working well, how much of an "advantage" do you need?

With the chain on the big ring pull it away from ring at 3 o'clock. If you can see day light between the chain and the peak of the teeth it's time to replace the rings.

Why?

After installing a new chain and cassete; felt grinding/vibration when in one chainring under load. Decided to replace the ring.

Great news! I just got a used one this year. Hope to beat that mileage myself.