I playback a video of about 18G using the Twonky Media Server via ethernet/DLNA.
When I select timesearch form the menu I can choose only form 0:00:00 to about 0:10:00, where there should be a bit less than 3 hours. Also fast forward with a factor of 8 or greater sometimes jumps backward rather than forward.
Experiments with different video sizes seem to suggest that the time wraps around at about 16GB of video size.
My firmware version is 1.55