Just my opinion of course, but I would shut it off once your pond temps drop into the upper 40's. I just don't see any benefit in running below those temps as there won't be much aerobic bacterial action going on, and of course there shouldn't be any shortage of oxygen in the water. So why pay an electric bill if you don't have to, along with wear and tear on the pump?

Please note I'm not saying Rex is wrong I'm simply saying this is my opinion.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.