Assuming:

1. Typical interview stress levels (I am watching)
2. Using familiar IDE and program language (their choice on their PC!)
3. Given adequate explanation and immediate answers to questions
4. Able to compile code and check answers / progress
5. Claims to be a senior level programmer

How long should it take an interviewee to answer FizzBuzz correctly?

Edit: FizzBuzz: Write a program that prints the numbers from 1 to 100. But for multiples of three print "Fizz" instead of the number and for the multiples of five print "Buzz". For numbers which are multiples of both three and five print "FizzBuzz".

Edit: It isn't so much that if they take more then X minutes they are disqualified, but I am curious if I should just cut them loose after they work on it for half hour.