Running in the 12.50 Index series (7 races), I changed it in March (before season), then succumbed to the temptation to change it after race 5. That was at roughly 5000.
I was planning to change just the filter and top off, but had the six quarts lying around and decided to change it since it's scarcely any more effort if you're catching used oil anyway.
This is my daily driver, as you can tell from the mileage. I certainly wouldn't go 8000 or 10000 without a mid-cycle filter change. That's just me.