How long and how high can an adult's fever be before they should see a doctor?

2-3 days or >102.5. Prolonged fever without a break for >2 days should prompt a call to your physician, and fever greater than 102.5 should also prompt a call. If the patient is sluggish or confused, take them to the emergency room (skip your doctor's office, as they will just send you on). However, if there are signs of pneumonia (productive cough with fever) or other bacterial infection - go in right away.