Is fluid change based on time or use?

[PDenbigh]

My schedule is very similar to Chad's. Bleed after a good track day (unless I'm fixing something else :evil, and flush at beginning and about 1/2 way through the season.

For oil, about once every 3-4 events depending on how hard I've run the car. That's with M1 15w40 and Blackstone says that I *could* go longer but that I'm safe with 3-4.