Welcome to ByC !
For ducks I stopped using heat by 3 weeks but it depends what the temp is
Mine was in summer and the house was warm
By then they were outside all day at 25-35 C degrees and would come in at night
I switched them to the duck house at 5 weeks as I don’t provide feed or water in the sleeping coop
I have a fully enclosed outdoor area I keep feed and water in that’s attached to the duck house
It’s all clear greenhouse panels to help keep it warm in the winter but keeps the weather of wind , rain and snow out so they can eat in the day
We get to -48 c some days winter where I live. My ducks wouldn’t come out to eat last year when it was that cold so we built the outdoor area it has a screen door and 2 windows one that has a fan to help with fresh air so it doesn’t get to humid in there
In summer it gets very warm but we added shade curtains to block direct sun and that really helped bring the temp down
They will come off as it gets cooler so it helps warm it up
They also have a duck door they can adventure out in the cold if they choose to
The coop is insulated with 2 windows for venting
I clean the pen daily and in winter 2 times a day to remove wet poo and don’t keep any water inside to prevent humidity. I will run a plug in heater when those temps got -48c to bring it closer to -20 as it’s just to cold