I have two of the Hydor 7.5s (one for my boy's @home tank, and the other for his here at school), for a 1½ and ½ gal, respectively. In my 1½, I don't have to unplug it, outside of cleanings - it cools down a bit during the day, and at night I cover it up so that it reheats. It's not a big jump in temp - usually about somewhere around 78º is where it stays, ±2º. In the ½ gal, it normally gets up to 80º before I shut it off, and that takes about 30 minutes.
I'm swapping out my ½ gal for my 1½ soon, and buying a 10gal - I came across one at Meijer for $10! - and I'll probably use both heaters at opposite ends of the tank. Thank the betta gods for power strips! ;)