You find the Microsoft Exam Policies at
http://www.microsoft.com/learning/en/us/certification/exam-policies.aspx.
There is no limit on the number of tries.
The first retake can be done after 24 hours.
The second+ retake needs 14 days waiting period.
Only up to 5 tries per year.
Once passed, you cannot redo the exam.
With kind regards,
Konrad