When are diamond blades for grinders considered dull?

[Stephen Tashiro]

I suppose that, by definition, a diamond blade for an angle grinder is considered dull when it doesn't cut as fast as the user wants. However, I'm curious whether these blades are designed to be used when the diamonds on the thin edge of the rim are worn off. After this happens, there is still a band of diamonds on the flat sides of the blade. Are the blades designed to be used in this condition?

[Tom M King]

When it won't go into what you're trying to cut, it's done, regardless of what it looks or feels like. How close to won't go at all is up to the operator.

[Andrew Hughes]

When I used diamond blades for cutting roofing tile we used them until they started hopping real bad. Sometimes that’s when they completely worn out sometimes it’s just a bad blade.
I could always make them cut faster by more pressure

[Steve Rozmiarek]

The edges heat up a lot more too. Don't worry, you'll know when it happens.

[Tom M King]

I buy the cheap grinders for this work, and use a small flow from a water hose directly on the blade, knowing that the grinder won't last. I'm not breathing any amount of that dust, don't trust a respirator, and don't want to ruin a supplied air hood. 30 bucks for a grinder is cheaper than going to rent a big one, and I don't do enough of the work to put a grand in one.

Sometimes, the small one is the best one for the job anyway.

[Bill Dufour]

When the tip recedes into the matrix. the matrix can be ground away to expose more of the stone.
Bill D