In my first few charging cycles of my v550, I did try to estimate the video energy consumption.
Unofficially (my testing conditons were not well controlled) I estimate that 10 sec of movie is equiv to the energy consumed by a still pic shot. So, somebody tested that the v550 could take 150~200 shots => 1500sec ~ 2000sec.
v550 Mpeg-4 is 0.5MB/sec, and 1GB is 2000sec.
So maybe (in my reckoning), you will finish 1GB of 640x480 movie without changing battery.
Practically you may try to save energy in movie mode by:
1. Turn off LCD
2. Turn off Continuous Focusing
3. Avoid optical zoom
4. Avoid fast moving objects to minimize mpeg-4 compression computation
99. Get an external 4xAA NiMH (1.25x4 = 5V) adapter to plug into the v550 5V input.
100. Prepare a pocket of ice to kool the DC which will get boiler-plate hot by the end of your movie.
And I think not too many v550 owners in this forum, are willing to test the video/battery limit for you, bcos that would depreciate the whole DC very much - overheat, SD life ...etc. FYI my clips are at most 5-minute (unless very important occasions).