Hello!
I actualy use a minn kota 55 lb trolling motor with my Hunter 23 when my outboard died. It was cheap, and I had been asking the same question without much response.
Short answer: it's exactly as it should be. Full speed under power is about 3-3.5 knots.
As for time until discharge? My range is about 4-5 miles at full speed, and probably double that at lower power levels. I have a 220 Reserve Capacity deep cycle 12V. Total outlay was ~ $275
Docking is awesome: i can coast right in withsails down, and then at the last second give the throttle a twist and turn on the silent brake (with some creative wiriing you can charge your batteries under sail this way). It can spin 360 degrees easily, as it's so light.
So, if you have to motor a long distance, don't go electric. If you need to just get away from the dock, buy the minn-kota and a nice battery. It will get you home as long as you make sure the battery's charged and you stay within your range. Maybe not fast, but you'll get there.
A 55 ft-lb is < 1 hp, btw.