I'd say no more than 4, but that's a personal opinion. Some might see it differently... But if you've spent more than a year preparing for the test and have taken it 4 times, then maybe it's time to call it quits, at least for a while (like a year or two).

I've also heard that some admissions officers don't feel that good about test-takers that have made this into a "hobby". Taking the test more than three times without any significant improvement might work against you in this case... But it really depends from school to school...

Thanks for giving me an answer.