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A friend said me that if you have one part in a very bad condition like a cassette and then you put a new stuff like a new chain it will destroy the new stuff very quickly. A LBS said that I should change my derailleur's pulley wheels after I said that the derailleur has undergone at least 10Mm. Then again, another LBS said that I should change only the cassette and the old derailleur pulleys do not matter. Hence, I am very confused. Are there any objective ways to state when one should change derailleur pulleys?

The diameters of the pulleys in the picture are about 3.6cm above and about 4.0cm below. They seem like the products here. Cannot find the specs of the original ones to compare, yet.

enter image description here

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Worn derailleur pulleys (also known as "jockey wheels") will not cause excessive wear on a chain because they aren't made of metal.

Eventually, though, the bushings inside will become worn and the outer teeth will wear down. Park illustrates the latter progression in their article on rear derailleur overhaul.

As with many things in bicycle maintenance the only objective metric for replacing something is that it no longer works satisfactorily. Their failure mode might best be described as gradual rather than catastrophic. If the pulley bushing doesn't have much slop in it and the teeth are still capable of guiding the chain effectively it's almost certainly fine. If the rest of the drivetrain is in new condition and there are still issues with shifting it's probably worth giving them a look.

As far as the practical matter of replacing them goes, in my experience for the most part you need to find the manufacturer specification to track down compatible replacements.

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    +1 that's a nice link. I agree that unless there's excessive tooth wear (not the case based on the pictures) or friction in the bearings (take off the chain and the pulleys should spin freely) then there's no point replacing these.
    – darkcanuck
    Commented Mar 6, 2011 at 19:54
  • the picture with pulleys of different life-time in the link is revealing, thank you. So when the pulleys are pointed, it is about the time to change the pulleys.
    – user652
    Commented Mar 6, 2011 at 22:08
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    The "doesn't work" measure I use is whether it still shifts. I have run pulleys down until there are no teeth at all on the bottom cog (I swapped the cogs around at one point) and the shifting just got worse and worse. At this point there's often so much slop in the parallelogram bushings that the derailleur needs replacing entirely. You can almost always get the generic replacement pulleys to work with 5/6/7/8 speed cassettes, usually with 9. I haven't tried with 10/11 speed cassettes.
    – Мסž
    Commented Mar 6, 2011 at 23:27
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Another failure mode of a pulley is that it seizes and stops turning. That happened to me at about 10,000 miles / 2 years. The failure was actually quite abrupt. All had been well pretty much forever, and then an odd noise as the chain slid over the seized pulley wheel and it resumed turning. Then about 5 miles later total failure. Tip: I removed the seized pulley, disassembled it, found that several ball bearings were missing and those that left weren't in good shape. So I took the balls out, re-assembled the pulley wheel with lots of chain lube, and rode it 8 miles home as if it was a plain bearing. Lesson learned: I now carry a spare pulley wheel.

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    Carrying a spare jockey wheel is overkill - I suspect it had been bad for a while and your maintenance might have been lacking in the leadup. So your answer is "before the pulley fails completely" Have a browse through the Tour in the Help menu to learn how SE is different, and welcome to the site.
    – Criggie ♦
    Commented Dec 29, 2016 at 6:45
  • A useful addition to this answer would be what is the make and model of the derailleur. In 45 years or riding bikes with derailleur gear I've never had this occur.
    – andy256
    Commented Dec 29, 2016 at 6:53