Continuous use of outdoor outlet
The first piece of information you will want to know is the power requirements of the heater. It is common for an electric "personal" heater to be about 1500 watts, which is also a very good figure for a blowing-type hand-held hair-dryer.

Rough calculation means that 1500 watts on a 110 volt home system in the USA will draw about 15 amperes of current. 

For this application, it is a good idea to have a dedicated circuit for the outlet that will provide power and use wiring and a breaker appropriate for 20 ampere current. Circuits are typically designed for eighty percent capacity. A twenty amp circuit would then provide safely sixteen amperes for the heater, but would have no reserve capacity.

No GFCI may not be a problem if you have no water contact involved (sheltered area?) but code may require otherwise.

You suggest that the heater will be on continuously, but a 1500 watt heater will generate enough heat that it is likely to cycle on and off to maintain a comfortable temperature.

Note also that many of these heaters will have low-grade power cords which may get hot and pose sufficient risk, regardless of the other portions of the circuit.



Designs such as the one pictured above will also have tip-off safety switches, although contact with oil-filled heaters are not as serious as contact with exposed resistance element type heaters.