There is no clear cut time limit for which a woman should wear a bra. It depends upon the size of the breasts, and the type of work one is doing. Main reason for wearing a bra is to give them support while working. Secondary reasons are to avoid them being obviously exposed, or to enhance the size of breasts in those who have small breasts or to prevent staining with milk in lactating mothers. I think whether it is 8 or 10 or 12 hours, it should not make much difference. However, you can consult a plastic surgeon for more information.