when to go outside?

5 weeks and they are feathered out sufficiently.

 

I put my chicks outside @ 2.5wks w/their heating lamp, they are now 4.5wks. It got down to 29 degrees here a few nights ago & they all (knock on wood) are doing fine. Currently, I still have their heating lamp on & their coop is fairly weather-tight, so it stays fairly warm.

As we speak, they are out in their in their run foraging around for goodies. Its around 55 degrees here & the sun is out

 

As long as the nights are above freezing and you have a large enough heat lamp and deep bedding for them to snuggle down in they should be fine.

I put my chicks out at just under 5 weeks, and it was still freezing at night. They did just fine and are still growing like weeds.

 

Silkies are a little different then most birds. The others have explained good time lines for regular feathered birds, and I dont know what kind of chickens you have but I thought I would add that these rules do not apply to silkies just in case.
They tend to need to be inside a coop more often then others for a longer span of time then most during cold weather. So make sure you have a way to block drafts and winds. And you should be fine. have fun enjoying your birds responcibly.