I agree with living2be,
Unless you really boil the fluid, or get it contaminated, every 2 to 3 years will be sufficient. Make sure that you choose someone who can bleed and purge brake fluid with ABS. - It's not just drain and pump up the brakes anymore.
Just looking over my Goodwrench Maintenance Schedule and I can't find replace brake fluid anywhere.
Now don't forget, your Corvette (read that RACING) friends, probably abuse the brakes more than average. And if you choose to keep autocrossing, then maybe you should listen to them a little closer.
Anyway, my $0.02 worth.....
Have fun with it!