Depends on how many degrees you're trying to raise the temperature and what setting you have it on. I have the same brand 1500 watt heater in my small basement office, on high it'll heat it up 3-4 degrees in a little over an hour, on low it takes longer. My daughter has the same model in her medium size bedroom in the basement, it probably takes a few hours to warm up the room.

The benefit of oil heaters is if you leave it on constantly they do a much better job keeping the room at a constant temperature. Mine started on low all the time, my daughter's stays between medium and low.