You don't say if it's a normal, sports or nursing bra. With a normal bra, you should wear your bra as long as you would any other under garment (singlet, camisole etc...) but a guide should be to wash it every 2 days by hand in a gentle hand wash product. If it's a sports bra, you should not wear it again until washed. If it's a nursing bra I would recommend you wash this every day as it would become soiled from milk leaks and could possibly smell. Never heard of infections got from dirty bras though.
