I think it really depends on your mode of transportation. In super cubs (airplane) for field work we update or capture every second. If by car, maybe similar. 4-wheeler or boat, maybe less often.
If walking, I think it would depend on how fast the person walks, and how smooth you will need the track log. Remember, it is a dot-to-dot (i.e. point-to-point). So, it would depend on what you are collecting and how you will analyze.
Just fyi: In our application, we actually capture points and create the lines on the fly. Because in the mountains we sometimes get a point that zips our to the "ozone", which will tend to crash the system (I calculate distance on the transect, and the distance overflows), I calc "speed" between points and throw out any that are at a rediculous speed (i.e 500 MPH).
I have an older project and, as mentioned, I create shape files, but still write the tracklogs to another directory, just in case the computer crashes during collection. The tracklog tends to keep data that the shapefile might not have saved to it yet. That is how I get around the write issue. My project is customized and written for 7.1....on my to do to make sure it works with 10.x for next year.
