Contineous Integration
How often continuous integration should happen?
is there any pre-requisite to enable this?
Are unit test cases should be automated?
The term is misleading, it should be "Continual Integration" and should happen every time a developer changes the code basis in order to make sure the whole system still works.
A pre-requisite for that would be a CI environment.
Have you ever tried to do unit tests manually? Can you imagine to do that with Continuous Integration?
Right, CI should happen with every check-in from the Dev team. And unit tests should be automated and should run with every CI.
The CI environment setup is a big task but once done it is really worth of effort.