There's no one answer to this question since brain fog can be a symptom of lupus that varies from person to person. However, there are some things you can do to help lessen its impact on your life.
Start by making sure you're getting enough rest and exercise. If stress is a contributing factor, try to find ways to relax and de-stress. See your doctor if the brain fog is interfering with your ability to work or socialize. They may be able to recommend medications or treatments that can help.