I'm with Greg, though I go a bit higher with 25-27K ADU. I have my Flats settings in Maxim set to 0.3 sec and adjust the brightness of my flat box to achieve the 25-27K. Any longer than about 0.5 sec and I take flat darks. For me using the SX cameras 0.3sec seems to be fine without flat darks.
ADU = Analog to Digital Unit. A conversion factor that converts your analog camera signal to digital units. These days most seem to be in the range of around 0.3 - 0.9 conversion rate (= gain. thanks Rick
). Short and simple, if your camera has a full well depth of 20,000 electrons to convert that to 16 bit digital units you need to divide your 20,000 by approx 0.3 to get 65535 ADU (or 16 bit). The deeper the well depth the larger the ADU conversion (gain) can be.