Brooder light/ ambient outside temperature

My crossing cross chicks are about 14 days old now. Is there a rule for taking them off the light in regards to how old they are? This is the first time raising Cornish during the winter. Lows and highs are forecasted as 36/71 right now.

Martin Bernal wrote: Is there a rule for taking them off the light in regards to how old they are? .

There are no rules my friend! If the space they have is large enough that the light is in one area, and the rest is unheated, they will self regulate, going to the heat when they're chilly and moving away when they get too warm. You'll know when to turn it off when you see them avoiding it and nobody spending any time under the heat.

Fall of 2020 I got chic by mail.  This was a first for me. Usually if I need chicks I buy them in the spring at my local feed store.  Because of covid-19 I wasn't able to do this.  I got my chicks at the end of September.  I learned the hard way about keeping them warm.  I had a chick die, who seemed healthy.  The only thing I could think was it got to cold.  I thought my setup would keep them warm enough, but I guess I was wrong.  I used a red heat lamp, and a heat emitter like you use for reptiles.  This setup worked great, and I didn't have any more problems.  
1 week old chicks need 90 to 95 degrees
2 week. 85 to 90
3 week 80 to 85
4 week 75 to 80
5, 6, &7  week need 70 to 75
8 week 65 to 70
9 week 65
Once chickens have all there adult feathers, they can usually keep themselves warm.
Some people who live in areas that get super cold  use heaters for adult chickens.  I live in California so once my chicks had all there adult feathers I took the lamps out.  Chickens are pretty good at keeping warm.   Where I live, I have to worry about the heat.  To be honest I will wait for spring from now on if I need chicks, it's so much easier.
Good luck to you, enjoy your chicks.

Jen Fulkerson wrote:
1 week old chicks need 90 to 95 degrees
2 week. 85 to 90
3 week 80 to 85
4 week 75 to 80
5, 6, & 7  week need 70 to 75
8 week 65 to 70
9 week 65
Once chickens have all there adult feathers, they can usually keep themselves warm.

Jen's chart is excellent to go by. I use it as my guideline, then, anytime ambient temp is the same or warmer, I turn the brooder light off until evening. I turn it back on when it starts to cool down.

Keeping warm takes a lot of energy for any animal. When they're little, they should be putting their energy into growing. Keeping them warm helps them channel their energy where it needs to go.

Rather than obsess over temperatures I look at chick behavior.  Are they all tightly huddled together under the heat lamp?  If yes they need more heat.  If they're all staying a long ways away from the spot under the heat lamp they're too hot and the lamp should be raised up or turned off.  If they're loosely grouped more or less around the area under the lamp then it's about right.

Obviously that assumes they have adequate water and food and clean bedding.  Any of those not being present will make their behavior not a reliable indicator.