I'll assume LabView here, and I've never used the C-Scan vi, but I have an (educated) guess.
It would likely be a matter of calculating the correct number of scans to hold in a buffer, since LabView sets buffers according to number of scans, not time or physical size. If each scan is so many bytes of data, and you want to hold 400 MB of data, then its trivial to figure out how many scans that is. Then wire the said number of scans to the buffer size input of the C-Scan vi. You'll likely have to change the number of scans to read as well, if you're generating data that fast. And of course, you'll be needing plenty of RAM.
M. J. Eggertson
Department of Chemistry
University of Washington