There's a good description of how to calculate the "pixel shift" due to field rotation on an Alt-Az mount as a function of your latitude, the position of the target in the sky, your sensor size (in pixels) and your exposure time here:
http://calgary.rasc.ca/field_rotation.htm
I have attached a spreadsheet which you can modify for your own location and equipment - if you can keep the exposure such that the "pixel shift" in the corners of the image is only 1 or 2 pixels in each frame, you should be able to get a nice series of frames which can be stacked successfully. 30-second exposures or thereabouts should be quite feasible for many targets - indeed, I generally get more pixel movement from telescope vibrations than from field rotation on my SLT mount.