10 frames per second seems to work ok for what we are doing (and you get more video for your money, so it is a bit more rewarding!).
I am using mencoder to turn the jpeg images from the camera into a mpeg4 video using the following:
mencoder mf://*.JPG -mf fps=10 -o leaves.avi -ovc lavc -lavcopts vcodec=msmpeg4v2:vbitrate=800 -vf scale=640:480
This produces an mpeg4 encoded .avi file, scaled to 640x480 pixels.
You can then run mencoder again to add a mp3 audio stream.
The results can be seen at http://flickr.com/jones139.