After about 200 miles on the track front OEM blanks look like that (that's their 4th day on the track)
Are these done for the tack use or is that just glazing cracking?

 

If you cant feel any actual cracks with your fingernails they likely have more time in them, but I would carry a spare set to the track with you just in case. Always a good idea to have a spare set on hand anyways so if one does crack your track day is not ended short just because of that.

 

Significant cracks that constitute rotor failure will have propogated all the way to the outer diameter during cool down. Depending on your conditions, the micro cracks form after a few sessions and will last a few more after so changing before an event because of micro cracks gets expensive. Because rotor failure is part of the game, we all learn to inspect the inner and outer faces after every cool down and always have spares. If your conditions remain consistant your eye will calibrate to how much life is left in the rotor. I wouldnt change a rotor prior to failure unless you want to avoid the scramble between sessions. Wear gloves to reduce cleanup time in the scramble. Even a rudimentary duct made from a piece of $25 shop vac hose from amazon and zip ties will in increase rotor and pad life substantially.