I have a good experience to tell.
Last friday when I got up I had some cabombas plants floating.
(is there a way to have them stick to the bottom definatly?)

Before I touch the water I always unplug the heater.
For some reason I forgot to plug it back in.

I have my 33Gal tank set up at 24C.

When I got home roughly ten hours later, my tank temp had dropped 2C
and it was at sitting at 22C.

So it does take awhile to lose alot of temp to affect the fish.

I am glad to say that no fish seemed affected by the temp drop.