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The refresh time is probably better then that if you apply some logic to it. For example, there should be at least three priority levels assigned to various gauges. RPM and speed should be refreshed every time, for example. The check-engine light should be checked too. That's three things that would be checked every time. All the other things, like oil temperature, etc, can be checked less often. Maybe once every 5-10 seconds. There's nothing that will happen that will make your water temperature spike in 5 seconds, for example, because the physical sensors have smoothing logic in them to keep the needle from bouncing around.
Finally, you can check all the rest of the stuff in increments every half minute or so. If it's not vital and doesn't change fast, you don't have to update it all the time.
The reason to monitor whether the check-engine light is on, btw, is simple. If it is, then something bad is happening, and that should trigger a new sequence of scans. If it lights up, your interface should first alert you, then scan every item on the OBD bus. That'll take a few seconds, but display the data as it comes over the pipe, and highlight any that exceed known good tolerances or are in an error condition. At this point, getting fast refreshes on speedometer/tach data is secondary to getting the whole picture.
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