Well, it depends. Every girl has a different nature. One may take a week to get comfortable, and the other may take a month. On average, a girl may take around two months or more to get comfortable with you. But you can tell whether she is comfortable around you or not by noticing her behavior. If she is sharing a lot of things with you, letting you know about her family and friends, or feeling happy around you, then she is feeling comfortable with you for sure.