Give the operator a front panel control to enter the Save Interval. Use a shift register on your While loop (initialized to a Double = 0) to hold the Last Time Saved. In your loop, call the Get Date/Time in Seconds function (from the Time & Dialog palette). If the current time - Save Interval > Last Time Saved, save the data and update the Last Time Saved.
See the attached LabView 6.1 example.