Paternity leaves are not mandatory. But it is paid as additional benefit. Some organizations offer it as they consider it at par with Maternity leaves. In fact it could be a good tool of employee satisfaction / employee engagement if you offer Paternity leaves.
Paternity leaves as far as my knowledge is concerned, offered from 3 days up to 1 Week OR 10 days. You may decide the number of Paternity leaves depending on the Company Operations, Average age of Staff and also the total leaves available.
These leaves should be paid leaves by management on submission of birth proof by concerned employee.