If you don't want to have to replace your bras more often than necessary, try rotating your bras every day, giving the band time to rest between uses. If you keep wearing the same bra day after day, before long it won't fit as well.
The Venusian Glow blog explains that the elastic in the bra band gets stretched out when the bra is worn all day long, and it takes time to recover from that stretching.
Another reason, the post points out, is that bras have different pressure and rubbing points, and rotating bras could reduced prolonged discomfort.
Why You Should Not Wear the Same Bra for Two Days in a Row [Venusian Glow]