Some women change their bra every two days, others change once a week...
This really depends on the individual. Some women perspire more than others, and their lingerie will need washing more often. Use your eyes and your nose to determine whether you need to change your bra.
Your bra will last longer if you wash it by hand. If you do prefer to machine wash, be sure to use a wash bag, never use wool detergents and never wash at higher than 85°F. You should never put your bra in the tumble dryer or dry it in direct sunlight or close to a strong heat source, like a radiator.
For 100% silk garments we recommend hand washing at 30 degrees with specialist silk detergent or dry cleaning. Do not machine wash or tumble dry. Iron to restore lustre.