I realize this is normal-- i.e., that under repeated hard use, these small hairline cracks emanating from the cross-drilled holes in the rotors really show up. Question is, at what point should I have the rotors changed?

Someone told me that when the cracks are long enough such that they're almost meeting, then I should go get them replaced.

Anyone have any ideas?

Rotors pictured are from my 996TT; stock rotors with stock pads. 28k kms (17.5k miles) on the clock. Pads have been changed at least once; rotors have never been changed.

 

If was me I would change these now, it's cheap and an easy job to do yourself! They say they will be ok until the cracks reach each other/holes but that won't be long looking at these! That's actually pretty good for 17.5k miles, I done 3 track days within 2 weeks and ruined a brand new set of disks on front, now upgraded.

 

The cracks look normal to me, however, as much as i can see from the pics the discs are worn out a lot and i suggest you to replace them.

 

So its a given that your rotors will eventually crack?

 

In my opinion, the time to change was yesterday!

 

i would change it

 

When you run your fingernail over the cracks does it get caught? If not, and you're not going to the track anytime soon, then you're fine.
When your fingernail gets caught is when it's time to change or whn the cracks meet.
Be good,
TomK