Rule of thumb is 90˚F the first week, drop 5 degrees a week till the brooder is the same as the rest of their world.
so, at 2.5 weeks, that's 90 minus (2.5 times 5) is 90 minus 12.5, is 77.5 degrees. They may be just about done needing the heat lamp as long as the house remains at 74 to 76 degrees. Do you term the thermostat down at night? If so, they may still need some heat at night for a couple of weeks. Otherwise, I think you may be done.
If they get bath - and I think it's good for them to have closely supervised bath time - no soap, just warm - 80 degree F - water up to the tops of their legs - then you might want to warm up the brooder to 80F for a little while after bath time to avoid chills.