The drains will stay in for as long as they need to. Usually this is for about a week maybe sooner or maybe longer it depends upon how much fluid drains on a daily basis.
This is a question you need to ask your physician who is doing your mastectomy as everybody's drain protocol is different. Usually if it is less than 25cc/24 hr period the surgeon will remove the drains and can be anywhere from 5 days to 2 weeks.