Paul,
Unfortunately, we don't really have a benchmark for this sort of thing since it is going to be highly coding and system specific. However, here are some suggestions to make the response time as short as possible.
*Increase system specs: faster processor, more RAM, less PCI bus traffic, etc. Hope this helps. Good luck with your application.
*Minimize other programs running at the same time
*Minimize network traffic, mouse/keyboard strokes, or anything that takes additional processing
*Coding efficiency: making use of NI-DAQmx instead of traditional NI-DAQ, proper use of locals and globals if necessary, efficient analysis
Overall, for a ballpark guess, it shouldn't take more than a few milliseconds to change.
