Well that depends on what else is going on with you (nausea, vomiting, pain), how long you've been running a fever, and what the possible and likely causes of the fever are.
A persistent fever over 100.4 for over 5 days without other symptoms warrants a doctor's appointment at least.
A temperature of 102 for a couple days in a row with no other symptoms warrants a visit to the doctor.
A temperature over 102 with other issues (headache, pain, nausea) warrants a visit to the ER.
Over 104 go to the ER, do not pass go, do not collect $200.
Over 105 don't stop to read the answers here just go!
Fever over 106 can cause brain damage and death, call 911 and get there by ambulance if need be.
What else is going on with you? What's your current temp? Have you taken it more than once?