Kyu exams every quarter, dan exams whenever sensei feels like it.
rokyu & gokyu--minimum 40 hours of mat time for each and instructor approval.
yonkyu & sankyu--minimum 60 hours of mat time for each and instructor approval.
nikyu & ikkyu--minimum 80 hours of mat time for each, one year between and instructor approval.
One year between ikkyu and shodan, two years before nidan, etc. You are "invited" to test for dan ranks when sensei feels you are ready.
No limit on how often you can test as long as you meet the requirements.