Too keep everything clean and sanitary I change my bed sheets twice per week but you can go as long as once per week without changing bed sheets.
However I've read online that some people go 2 weeks or even a whole month without changing their bed sheets and washing them but I think that's too long to wait but that's my opinion.
You really should wash and change your bed sheets a minimum of once per week since when you're sleeping you get bodily fluids such as sweat on your bed sheets and even mattress even though cannot really see it.