I connected Duet to MacBook and let it play music through iTunes for about 1.5 hours. FYI, MacBook was plugged into an outlet.
For the most part, my fan speed hovered around 4500 rpm while Duet was connected, occasionally going as high as 5200 rpm and low as 4000 rpm. For a point of reference, the fan speed is usually around 1800-2200 rpm. The temp also went up about ~15 degrees as well. The fan noise was clearly audible and distracting, especially during quiet passages on music.
I've connected the Duet back to iMac and I've let it play some music for about 40 minute now with no real changes in any of the fan speeds.