A cardiologist can have some varied hours. Cardiologists generally will only work during office hours, but will NEED to be willing to work out of the hours of the clinic, if consultations are needed in the hospital.
An advantage of being a cardiologist is that this career pays well. Another advantage of being a cardiologist is that you can save a person's life. A disadvantage of being a cardiologist is that you work a lot of hours. Another disadvantage of being a cardiologist is that you can not cure everyone.
Ignoring statutory holidays and the variation of days in a month, you could work it out this way:- there are 52 weeks in a year - thus 30*52 = 1560 hours a year. there are 12 months in a year - thus 1560/12 = 130 hours a month.
It depends a lot on who you work for, and what their policy is. You typically accrue vacation hours on a monthly basis, whether it's 4/month, 6/month, or 8/month. That info will be in the contract that you signed...