I have that heater in one of my tanks, and to be honest I don't like it because it's inconsistent. Sometimes it'll heat the water up quickly and sometimes it takes hours, and then I have to turn it off at night unless it's really cold, because it'll sometimes make the water too warm. If I ran the heater constantly, the water would get dangerously hot.
I do remember that later when I bought a different brand of heater, it said on the packaging that the first time it's used, it'll take longer to warm. So I wouldn't return the heater, just make sure you keep a careful eye on the temperature so that you know when the heater needs to be turned on and off.