Mine are almost 4 weeks old and it's been so hot here I haven't used a lamp. You can definately tell if they are uncomfortable though. Sounds like you are doing fine!
We have 50 Rhode Island red and Barred Rock chicks in a 4x4 brooder in a Morgan building. They are currently 13 days old. They were in with 30 ducks the first week also. With the daytime temperatures in the 90s the first week and the temperatures this week in the 100s we have not needed a light. It's only been dropping into the mid 70s at night. I only had 100 watt bulbs on everything for the last couple of months as well.