I change my bed sheets every week and replace them with freshly washed bed sheets.
Some people say you should wash the bed sheets once every 2 weeks which is usually enough but I don't leave my bed sheets on my bed longer than a week without washing them and replacing them with new ones.
It may be excessive but I sleep better knowing my bed sheets are clean.
As long as you replace your bed sheets with fresh clean ones every 2 weeks then you'll be fine.