when can I kill the light?

Where do you live? The temp without it is whats important. During the day here it has been very warm/hot so I have been turning my light off. I turn it on at night but put a 100 watt bulb in instead. Mine are 3 1/3 weeks and doing great.

 

I know - I put a 100 watt bulb in mine because the 250 was so hot. Just put it closer to the ducks. Then went down to a 60 watter.

You do not need the 250 now. Really depends on the temp in your barn.

I put my ducks outside at four weeks. At night i locked them in a dog box with hay. They are doing fine. Temperatures were 40 - 45 at night.
Now they go in the box to sleep on their own.

I do not blame you for worrying about the 250 watt bulb in the barn. I would kill it. Someone on here wrote that their ducks starting feathering faster after they got rid of the light. I noticed this with mine also. I do not want them to be cold but they huddle and have plenty of hay.

 

for me it depends also on how many chicks I am trying to wean at once. I usually put a cardboard "barn" in their cage, deeply bedded with bermuda hay. If there are enought chicks they go in there at night and just snuggle and keep eachother warm. I throw a blanket over the cage at night also to cut drafts. If it is going to be cold, which is a relative term, I know, I will bring them back in at night and put them under a 75 watt bulb for the night only.

 

By all means get rid of the light! Ducks do better in cooler weather than they do in heat, the bigger the bird the better to deal with cool weather.