I image with SBIG cameras. What exposure length I choose to use for HDR purposes is a function of many variables, including (i) how long my other exposures are, and (ii) how much dynamic range I'm dealing with.
1. When I shot the Orion Nebula with my FSQ106, I used exposure lengths of 15-seconds, 90-seconds, 300-seconds and 600-seconds, because of the extreme range of brightnesses in that object.
2. For my recent globular clusters, with my f/9 14.5" RC, I took images of 1-minute and 5-minute durations.
3. For my recent M81 with the RC, I took a series of 15-minute and 5-minute images.
If you're worried about not swamping either background noise or read noise, you're taking exposures that are too short for the brightness of your object, IMO.
I dither between every subexposure.