They should be able to go out without a heat source when they are fully feathered at 6 weeks of age, so not too much longer.

I agree with Audrey... Once they are well-feathered they are able to retain their body heat better. If your nightime temperatures hold, it won't be long until they can manage.

Even so, if you get occasional cold nights (I don't know your zone or weather patterns, Claraserena, but we have recently had night temps as low as 50 degrees F.) you may need to use a light for a bit or at least throw an old blanket over a box for them and provide extra bedding.

An alternative to a high energy heat lamp, when less extra warmth is needed, is a "coffee can heater." Place a light bulb (40 or 60 watts) in a ceramic fixture suspended inside a coffee can. Add a cylinder of chicken wire inside the can but around the bulb and cord to keep the chicks from getting at it. Place it in their house, anchored to a non-flammable surface, and the chicks can cozy up to it like folks round the stove in the old general store. If you want them to have a "night light" while they adjust to nighttime darkness, punch a few holes in the sides of the coffee can before you start.

The first year I had chickens I used a variation of this to keep their water from freezing. It worked fine, but the heated dog dish I have now is more convenient.

Thanks Maggie for the coffee can idea. I'd feel a lot better knowlng they have a way to get warm.
Just a note--I am surprised at the range in size of the chicks--even among the same breed--we have one Speckled Sussex that is half the size of the others--she seems healthy but is just small--kind of a short body. One reason I want to get them into a bigger space is so they can get out of each others' way. Meanwhile, I've given them pieces of turf, lettuce, dandelion greens and watermelon rinds to keep them busy.